If you want some guidance on how to use the Gimp for mapping, you have come to the right place. The best place to start is with RobA's "little" tut that you can find http://www.cartographersguild.com/showthread.php?1142-[Award-Winner]-Using-GIMP-to-Create-an-Artistic-Regional-RPG-Map. It is a great tut to learn the various aspects of Gimp, and a lot of people have used it to springboard into their own style by tweeking the methods learned from it and other tuts. If you have any questions on it, just post them in that thread.

Once you start the tut (or any others that you will find in the Tutorial section at the bottome of the main site page) start up a WIP (Work in progress) thread and ask any questions about looks or ways to improve the map. Don't be shy, 'cause we are a freindly bunch who's only real fault is our addiction to great maps (and helping create the same).

Welcome and have fun.