If you have downloaded Gimp, then the best way imoho, is to look at RobA's tut on creating a regional map. It is a large one, but it teaches you soooooo much about Gimp and its abilities. Run through that one and a few others then post the results and people will let you know what they think may improve it. After a while, you will be creatingmaps in styles that you like.
Have fun and post maps as often as you can