Do your research, ask questions about different tanks, setups, running costs etc.
Visit lots of different aquatic shops, (a good shop will have plenty of time for you, even if it means you have to go back and visit them again)
Look on the internet, find out info from other people about problems, time you will have to spend looking after the tank. (the fish don't feed themselves)!
Keeping fish in general (though they are relatively low-maintenance pets) necessitates some basic knowledge of requirements and patience, and this goes doubly true for marine animals, which are usually many times more sensitive to water quality than their freshwater counterparts. So there is some degree of minimal devotion that you really must have and keep up with to 1) do the background research required and 2) determine if you have the time and financial resources to maintain a healthy marine system over the years.

EQUIPMENT
The type of equipment you get will depend largely on what you are really aiming for in terms of livestock. You must decide on what you are most interested in keeping, and then do research on those species to make sure they are suitable for beginners (many are not).
With a Fish Only system this will tend to be the easier type of marine setup as you can in many cases just go with a standard tank setup and the equipment provided, with a few add-ons, like protein skimmers, and powerheads. Within Fish Only there are two primary subtypes... the peaceful "community tank" (usually houses smaller, more omnivorous fish like clownfish, damselfish, gobies, small wrasses, dottybacks, etc., which may still be territorial and must still be carefully matched) and the "aggressive or semi-aggressive tank" (which houses fish such as lionfish, triggers, various eels, groupers, and usually larger, predatory fish). You cannot generalise which fish will always work with which other fish, because this is a tricky subject depending on individual combinations. Clowns, for example, are often considered "peaceful" fish, but there are species that are more aggressive than others, and some will kill those of similar appearance unless they are a mated pair (and no, clowns do not NEED an anemone host, contrary to myth--they can have a symbiotic relationship with one, or can live alone--anemones require high lighting and water conditions, and are not recommended for beginners). Lionfish, as another example, are peaceful towards animals their own size, but will eat just about any moving animal that can fit into their mouths. And tangs and angels, which are both gorgeous and popular groups of fish, may be seen occasionally in both types of tanks, but require a lot of swimming room and good water movement.

WATER MOVEMENT
Enough cannot be stressed about good water movement in your tank. We have learnt the hard way that lack of water movement means lack of oxygenated water and fish death.
Saltwater fish come from areas where the water is in constant motion. One of the easiest ways to accomplish this is through the use of powerheads. A powerhead is simply a water pump that can be placed in the aquarium to create water movement. The use of multiple powerheads positioned in different areas of the aquarium can also create random and chaotic flow. Another benefit to the increased water flow is it will cause debris to stay in suspension in the water column allowing the filter to trap the debris. One last benefit is that the extra water movement will allow for better gas exchange between the aquarium and the atmosphere. By creating more movement on the surface of the water, it will allow carbon dioxide to be released from the water into the atmosphere and oxygen to enter the water from the atmosphere.

WATER
A basic understanding of water chemistry is extremely important in properly maintaining a saltwater tank. This is why people who have never kept any type of fish before may be better off starting with a simpler freshwater tank first before tackling marine, just to acquire very basic experience, such as with the nitrogen cycle. Marine animals are more sensitive than freshwater fish to dissolved solutes and impurities, such as those usually found in tap water. Freshwater animals may not be affected by anything except chlorines and chloramarines in tap water (removed by dechlorinator and stress coat type products). For marine animals, even tiny ppm (parts per million) fractions of impurities may be. The only way to really get rid of these is through water purification.
This water should then be made up to; Specific gravity (S.G.) measures salinity, or actual concentration of the salt... for marine animals, this should fall in the 1.021-1.024 range. S.G. should be measured with a hydrometer. Other tests which are absolutely vital for beginners to own include ammonia and nitrite (both should be 0 at all times in a cycled tank) and pH (should be roughly 8.3-8.4). pH can be kept stable through addition of a marine buffer, and in water changes or anytime there is life in the tank, this should all be done in a separate bin and premixed with the salt. Other good tests to have that are not as vital include KH (to measure buffering capacity, which determines pH), nitrate and phosphate (both best kept at a minimum for optimal coral calcification, and to prevent pest algae from blooming), calcium (best kept as close to or above 400 as possible), and copper (absolutely needs to be kept at 0 in all reef tanks; it is an ingredient in antiparasitic fish meds and sometimes seen as a trace in tap water, but is highly toxic to all inverts). Some corals may require dosing of certain trace elements (calcium, iodine, strontium, etc.) and some filter-feeding organisms may need separate feeding with phytoplankton.
Also remember that when just topping up the water in the tank only use deionised or R/O water. Do not use tap water unless you treat it with a suitable tap water treatment product.
Do not top up with saltwater as this will increase the salinity in your tank, when water evaporation occurs the salt levels within the remaining water stays the same. Hence, only needing to do top-ups with unsalted water.
Obviously, when doing a water change use saltwater.

PROTEIN SKIMMER
We have had many problems with our protein skimmer including a couple of 5 gallon floods over the kitchen carpet. It still doesn't produce as well as we would like it to but we have discovered that if we use just R/O water the skimmer stops producing waste, if we add just a small amount of Aquasafe to the sump the skimmer starts to produce again.
We did run the tank for a couple of months without a properly working skimmer and the only effect we noticed was that we had to change the external filter media more regularly and also more water changes were necessary.
The protein skimmer is a major piece of equipment to help you achieve pristine marine water. It is a type of prefilter that can be used with conventional biological filtration
It works by generating tiny bubbles that rise up through a plastic column and into a collection cup above the surface of the water.

As the bubbles rise, their outer surfaces collect sticky proteins from the water, removing them from the system along with the potential ammonia, nitrite and nitrate that they would otherwise be converted into by biological filtration.

ULTRA VIOLET STERILISERS
These can come in useful as their main role is to cut down parasites and pathogens in the water.
Water passes through the unit and is exposed to the harmful light within. The light is separated from the water by a clear, quartz tube that may need cleaning occasionally. The units connect via hosetails to an external filter or sump pump, and the bulbs should be changed every six to 12 months, depending on the manufacturer.

Tanks containing fish that are susceptible to whitespot will really benefit, and they can help to cut down algae and clear the water.

FILTRATION
We use an Eheim Pro 3 external filter. When we first set the tank up we just relied on the alfagrog and protein skimmer on the system but then realised that this wasn't doing the job we wanted. The external filter we use has four baskets inside for different media. In the very bottom basket we use Eheim Mech.
This is a course material used where water enters the filter process. Water is swirled and coarse particles of dirt sink. Then the water continues its way through the other layers.
The second basket is filled with Eheim Substrat Pro which is biological filtration. This product offers optimum colonization conditions to as many purification bacteria as possible.
Due to its pearl-shaped structure, the bulk density of SUBSTRATpro is very high and it is nonabrasive. The filter volume is used to its fullest extent and no useless empty areas are formed, in which filtration cannot take place. The macropore structure of the SUBSTRATpro balls is designed so that the important purification bacteria find large, easily colonisable areas.
We then have a basket filled with JBL Phosex. This is a phosphate removing product. Together with nitrates, phosphates are the main nutrients for algae. Excess phosphate levels therefore encourage the growth of unwanted algae. The main source of phosphates is fish food.
The very top basket is filled with Eheim activated carbon. Activated carbon filtration is one of the most effective and easiest methods of removing organics from aquarium systems. Our reef and fish-only aquariums are NOT miniature slices of the ocean. They may look that way, but bio-chemically they are an ecosystem that is always on the verge of collapse. Activated Carbon’s job is to remove metabolic wastes, or more commonly called organics
It removes odours, removes discolouration, and makes aquarium water as clear as ice.

FISH
Many books will show you compatibility charts that say certain fish will not go with any species of fish or say take caution when putting these certain fish together.
In some cases this is true, such as putting small fish with a lionfish, the result will be that the lionfish will have a very expensive meal.
However, some reference books and fish shops will tell you not to put more than one butterfly fish in a tank as it will fight with other butterfly fish, we have found this not to be the case and have had many butterfly fish in the same tank together. We also have two tangs living in the same tang happily as these are from different genus of tangs. Triggerfish have a reputation of being aggressive but our blue trigger is very peaceful and quite shy. Again use compatibility charts as a rough guide but do ask your local fish shop for sound advice and use some commonsense. If you already have fish of a substantial size in your tank when buying a new fish try and get that isn't too small and is proven to be a good feeder. Before you buy any fish ask to see it being fed in the shop.

GARLIC AND VITAMINS IN FOOD
The product we use is Seachem’s GarlicGuard™ which contains the naturally derived, active ingredient found in garlic, allicin, which has been demonstrated to possess health promoting benefits. Allicin possesses strong anti-oxidant properties (similar to Vitamin C) which promote enhanced health through elimination of dangerous free radicals. For enhanced health benefits, GarlicGuard™ also contains Vitamin C.
We also use JBL Atvitol which protects fish against deficiency diseases, enhances the brilliance of colours, encourages spawning, increases appetite and promotes growth. The addition of JBL Atvitol to aquarium water provides the microorganisms necessary for the breakdown of biological pollutants with vital active substances. This provides the best conditions for biological filtering.

FISH FOOD
It is advisable to research the sort of fish that you would like in your tank and then to make sure you know what sort of food they require to stay healthy and to provide the nutrients they are going to need. We are lucky that all our fish are good eaters and are not fussy feeders. We do buy specific frozen foods, e.g. Angel and Butterfly frozen cubes for the angel and butterfly fish, Mussels/Shrimp for the triggerfish. We also make sure we have a good supply of algae sheets. Make sure you buy the best food that you can afford as it isn't worth trying to cut costs in this respect. Cheaper foods will not provide your fish with everything they need and in the long term if your fish are not getting the nutrients they need they may well stop feeding.

WATER TESTING
It is essential to test the pH level of your water, Ammonia levels, Nitrite levels and Nitrate levels.
pH should read 8.3, Ammonia and Nitrite 0 at all times and Nitrate under 25ppm (preferably 10ppm or less.)
We also test the following, O2 (oxygen), Phosphate, Calcium, Magnesium, kH (carbonate hardness) and very occasionally Copper.
pH, Ammonia, Nitrite and Nitrate are tested once weekly.
Phosphate tested fortnightly.
All other tests are done once monthly.
An optimal level of magnesium in a saltwater aquarium is required in order for invertebrates and ornamental algae to thrive. Just as in nature a saltwater aquarium should maintain a magnesium level of c. 1300mg/l (ppm).

FRESHWATER DIP
This is exactly what it sounds like, water with no salt. When we have noticed any flukes on the fish (which hasn't happened too often) it is time to give the fish a dip. We make up a small plastic container with water at the same temperature as the tank with the same PH and use Aquasafe to get rid of contaminants. You then catch the fish and put it into the freshwater. It is safe to leave the fish for up to 5 minutes if needs be. After a couple of minutes it is easy to see flukes come away from the fish. If at any point the fish appears to be stressed (breathing difficulties, trying to leap out of the container) then introduce it back into the main tank. Tangs will normally lie on their sides for a couple of minutes and appear to be dead but don't worry they soon recover. Boo has had a few freshwater dips and each time he has been fine.

CALCIUM
The ideal calcium level should be between 350-450ppm.
In the ocean, there is a constant source of vital minerals, nutrients, and vitamins. In a closed aquarium environment, some of these essential minerals are depleted as corals and invertebrates use them to grow.
Aquarium additives replenish essential mineral and nutrient levels to provide an ongoing source invertebrates need for proper growth, physical strength, biological function, and coloration. They also help the organism resist minor changes in water conditions, as well as diseases. Thus we add calcium when needed.