I learned Photoshop in two different ways. The first was tutorials. Lots and LOTS of tutorials. They are still probably my favorite learning tool. There are some really cool things you can learn to do if you just follow other people's steps and in doing so you will learn your own techniques. The same could be said for GIMP. Other than that, just jump in and get your feet wet. Its a matter of starting something and when you get stuck (generally within 5 minutes
) then using Google to find your answer and moving on.
I only used GIMP for about two weeks before I decided I liked it so much I would buy Photoshop Elements and then I used it for a couple of features that it had that Elements couldn't do. So I don't have much experience with it at all, but those links should help you get started with things.