Photoshop is a very powerful image creation/manipulation package. You will probably use it every time you bring an image into your computer, whether it's your own scanned or photographed work or stock footage, or whenever you need to create images directly within the computer.
The programme is used throughout the entire industry for print, film, animation, web and 3D work - it really is a "big" one... and given this depth, accept that you will never know the programme in it's entirety, and neither should you.
Hopefully, by the end of the course you will have a basic understanding of what the programme can do, and specifically what it can do for your work.

Here is a four stage process you should follow when working on an image...

Four stage process:
Open and assess use of the image (image/image size)
Set the tonal range (image/adjust/levels)
Play (composite, colour correction, filters etc.)
Save (one photoshop file for archiving and at least one other format for distribution)

In detail then:

Open and assess use of the image (image/image size)
When you first open an existing image or create a new "piece of paper" to work on you should open the image size dialogue box.
Here you are assessing or deciding on what the final destination of the picture might be. The box tells you how large the file is (the larger the file, the harder the computer has to work to change it), what size the image will physically print out at, and the resolution of the image.

There are four resolutions that you should remember:
72ppi [for any image to be seen on screen]
150ppi [for all types or low grade paper publications]
300ppi [for commercial offset-litho printing]
360ppi for inkjet printing onto high quality photo-grade papers

Unless an outputter specifically asks for a different resolution, one of these will be fine.The image size dialogue box is used for juggling resolution and print size, ensuring that the image has good detail, prints at the right size and is manageable by your hardware.
NB. because you are able to change deep level information about the image in this dialogue box, it is possible to do real harm to it, so always keep an original copy just in case!!

Set the tonal range (image/adjust/levels)
Opening the levels dialogue box presents a graph representing the tonal information within the image. Underneath the graph are the three sliders which are the only bits that you need to take any notice of. The slider on the left (a little black triangle) sets the "100% black point" [ anything to the left of this slider in the graph becomes completely black]; the slider on the right (a little white triangle) sets the "100% white point" [anything to the right of this slider becomes completely white]; logically enough the middle slider sets the "mid or grey point" of the image.
Typically, a digital or poorly lit or scanned photograph will show a graph with most of the information bunched in the middle ie. little or no blacks or whites. To fix this just play around with the sliders [generally, if you pull the black point just under the first peak of the graph, pull the white slider under the last peak of the graph and leave the mid point to look after itself the image looks much livelier].
This is your really your first artistic decision within the computer - it's up to you what settings create the mood you're after.

Play (composite, colour correction, filters etc.)
Remember that there are always at least five different ways to do the particular thing that you want to do - "if the method you know doesn't work within a couple of minutes it's probably worth looking for one of the different methods".
Photoshop rewards "touchyfeely" exploration and it's up to you to find which bits work for you.
However:
You'll almost certainly want to do compositing [combining different image elements into one picture]. Probably the best way is to place each element on a seperate layer [layers are "infinitely large sheets of transparent cell" that can be dragged independently around the image] and then EITHER use the eraser OR apply a layer mask (layer/add layer mask/reveal all) to hide or reveal parts of the indivdual elements.
To seat seperate image elements you'll definately want to experiment with the free transform tool [select the part of the image to transform/edit/free transform]. This tool allows you scale, rotate, distort, put into perspective and generally mess around with imagery [right-clicking within the active transform area brings up a list of the other transform tools, to accept the transformation when you are happy with it, double click within the transform area].
There are many more than five ways to work with colour adjustment in an image. The most "hands on" is to add a new layer (layer/new/layer), set the blending mode [the blending mode determines how elements on different layers interact with elements "below" them] of that layer to "colour" (window/show layers/click on the dropdown box that is set to "normal" by default and set it to "colour") and simply paint your new colours.

Save (one photoshop file for archiving and one for distribution)

Images can be saved in a huge variety of formats, each format having it's own particular strengths...
However:
Always save your work as a .psd (file/save as) as this is the only format that maintains all the layer information, enabling you to change the compositional elements of the picture at a later date.
Then:
If you intend to take your image into a page layout programme such as Quark Xpress or into Word then you should also save a .tif (file/save as/format dropdown box: .tif).
If the image is to be put onto the web you should save a .jpg (file/save for web/format dropdown box: .jpg).

Finally, there are many excellent books, CD's, websites, magazines etc. all devoted to helping you get the most from the programme.

Now go Play...
Oh, and don't forget to use the modifier keys and as many keyboard shortcuts as you can: it really does speed up your "workflow".

when using with an active selection:
shift key - adds to a selection
alt key - deletes from a selection
control key - cuts the selected area
control and alt keys - duplicates the selected area
control and d - drops any active selection

painting tools
alt key - temporarily accesses the colour selection tool
shift key - creates straight lines between mouseclicks or constrains lines to cardinal angles
spacebar - accesses the hand tool
spacebar and control key - accesses the magnifier
spacebar and alt key - accesses the reverse magnifier