Man... you bought a copy of CS4 just to learn how to use it? I congratulate you on taking such a plunge, I don't have the nerves to do that.
Learning to do basic stuff is easy. Play with all of the tools in the normal toolbox to see what they do, and then Google them for examples of what can be done. For example, load up a personal photo an experiment with touching it up. Use the dodge tool (mid tones, 5-10% effect) to whiten teeth. Use the spot healing brush to remove scars or blemishes. Use the burn tool to give yourself a tan, etc. Obviously it won't come out very good, but it gives you some concept of what they're used for.
Experiment by creating embossed buttons, such as for a website. You can find tons of great guides out there that will tell you how, and most are very easy to follow step by step.
Practice using keyboard shortcuts to select, deselect, zoom, etc. Once you get used to some of them (I'm still learning myself), it'll make life easier.
Realize that saving a compressed or final image (JPG, GIF, PNG, etc.) is completely different from saving a PSD. PSDs aren't designed for normal viewing; they're meant to be a workable product that will retain layers, guidelines, cuts and other information that would normally be discarded. So, saving your work as a PSD will allow you to go back and keep going from where you left off.