Welcome to the Guild, the Mappiest Place on the Web
There got that out of the way
Where to start learning to map with the Gimp....
Step 1 - Join the Guild.....check!
step 2 - Check out Tutorial forum.....specifically look for this one as a starter. A long read but well worth it to get a handle of the basics of the Program.
Step 3 - Work through the tut a few times to get comfortable with the prog.
Step 4 - Post up you work as you go in the WIP (Work In Progress) Forum. Postinng your work here is the BEST way to improve. It is where you can ask any questions you may have and people are very helpful with plenty of suggestions. Don't worry, we don't bite.
Step 5 - Post you finished map in the Finished Maps forum for people to admire
Step 6 - Rinse and Repeat
There THAT should get you moving in the right direction.