i learned some basic stuff in gimp and lately i tried photoshop cs3 and found many similarities so i learned basic stuff on other app on other os that worked in it, still find more simple and capable gimp as a image editor
there was a post on this forum with many tutorials in video to help you learn gimp, there is also a forum with lots of tips to use on gimp, it is a not that complicated app with so many capabilities, can be a little scary but is similar to other apps, you don't use everything on it, only what you need, it is a good approach to the app, do only what you need and learn slowly other options when you require them