Thanks for the rep!
Two years ago I spent about two or three weeks playing around with GIMP over Christmas break; I think this is really the best way to learn the basics of the program. I did a walk through of Ascension's Atlas style Tutorial (which Gidde had done a GIMP conversion for), and this gave me some good practice. I'd recommend doing something similar: find a tutorial that you like and then just practice running through it a couple of times in GIMP. Anyway, hope you enjoy yourself.
Well it's great to see another fellow OPer here at the Guild! This is such a great community by the way, I knew literally nothing about digital map making, or about digital art for that matter, before coming here. The good people here helped to get me on my way though. I hope that you enjoy this community as much as I do. Let me know if you have any questions about anything.