Lots of a tutorials. Lots and lots of tutorials. Even if its an effect you dont want, youll see tools that you can use differently and how to use them in unique ways.
Kamikaze is right, tons of tutorials, and learn tutorials for everything, no matter how simple it is, Gimp is a pretty robust program, and I like it better then Photoshop (IMO) but it's much more difficult to use given the circumstances.
what kinda of things do you want to learn i have a lot of experience with gimp, PhotoShop, Illustrator and InDesign. and can slap some tutorials together.