Great, you have come to the right place. First off let me just say that I am bran new to the guild myself. In fact I only just learned how to use GIMP (on the fly) over the Holidays as I worked on my own map. Hence, my own knowledge and expertise being negligible, allow me to direct you to the resources that I found helpful here. The first thing that I did, was to search through the "mapping"section to get a feel for the various different styles of maps that were out there. Next, I went to the "tutorials" section in search of GIMP map-making walk-throughs. In this regard I found the following tutorials extremely helpful (Thanks RobA, Ascension and Gidde!):

http://www.cartographersguild.com/showthread.php?1142-[Award-Winner]-Using-GIMP-to-Create-an-Artistic-Regional-RPG-Map

http://www.cartographersguild.com/showthread.php?4084-[Award-Winner]-My-atlas-style-in-PS/page15

Both of these tutorials will help walk you through the process of map-making in GIMP; however, if you are unfamiliar with GIMP, I highly recommend reading through the GIMP user's manual online. Also, RobA and others have developed a number of Scripts for "pimping" your GIMP that you really ought to check out. These scripts and plug-ins are posted throughout the tutorials section. Also check out the Gimp plug-ins registry:

http://registry.gimp.org/

to pick us some helpful plug-ins (you really should get the "Layer Effects" plug in if nothing else). Well this is enough to keep you busy for a while. Hope this helped somewhat.

Cheers,

-Arsheesh