http://www.gimp.org/tutorials/
Go through those, and that should give you a good basic grasp of the software. Once you've done that, check out
http://gimp-savvy.com/BOOK/index.html -- it's an online book that goes through everything in detail. If you get really stuck or have a question that those sites don't answer, send me a PM. I use GIMP sometimes for work, so I have to know it reasonably well.