Start at the beginning... Check out tutorials here: http://www.cartographersguild.com/forumdisplay.php?f=48
Specifically, start with RobA's GIMP tutorials here:
http://www.cartographersguild.com/showthread.php?t=1142 (10000+ views!!!)
and here: http://www.cartographersguild.com/showthread.php?t=594
and here: http://www.cartographersguild.com/showthread.php?t=875
and here: http://www.cartographersguild.com/showthread.php?t=533
and here: http://www.cartographersguild.com/showthread.php?t=1048
and here: http://www.cartographersguild.com/showthread.php?t=874
ummm and here: http://www.cartographersguild.com/showthread.php?t=1397
Torstan's hand drawn GIMP maps here: http://www.cartographersguild.com/showthread.php?t=2035
Hapimeses Mountain tutorial (external link to video tut) here (not GIMP specific, but general enough to use almost any software with): http://www.cartographersguild.com/showthread.php?t=1669
These posted by pyrandon (Photoshop, but should be generic enough to do easily in GIMP): http://www.cartographersguild.com/showthread.php?t=547
and here: http://www.cartographersguild.com/showthread.php?t=1482
and here: http://www.cartographersguild.com/showthread.php?t=1150
Then after all that, you can look at any others that kind of strike your fancy. It all depends on the style of map you are looking for. Also, be sure to check out the featured maps here: http://www.cartographersguild.com/forumdisplay.php?f=43
And finally, start mapping and post your works in progress using the manage attachments button as as you go. I would suggest an order like this for outdoor maps (regional and world):
- Land and Water (Seas and perhaps lakes)
- Mountains
- Trees
- Rivers
- Towns
- Roads
and post between each session for feedback.
Joe