That's what's funny about GIMP and Photoshop- they do almost all the same things, but after getting familiar with one, it's near-impossible to jump to the other, because the interface is all different. So, basically, you have to learn the whole thing from scratch again. That's really how it works. Sorry.
To speed up that process, however, I have a Guide you can read. Made it a long time ago and my Photoshoppery has increased quite a bit since then, but it still covers the basics. PM me if you want more help with it after that- I don't mind teaching.
Also, sorry that the images in it are ginormous. I made it back when the site actually auto-resized those things.