I'd recommend poking around the Finished Maps section to see if you find something that you like and then see if there is a tutorial for that kind of map lurking in the Tutorials section. There are many ways to get things done, including straight art programs like GIMP or Photoshop, straight GIS programs (too many to mention), and more specialized tools like Wilbur or Campaign Cartographer 3 / Fractal Terrains. The first step is to decide what you want on your map, including a very rough and ugly sketch. The next step is to decide how you want it to look (the Finished Maps section here is an excellent resource. The step after that is to look through the Tutorials section and see if there's anything that will help you use your tools to help you get moving toward your goal. The last step is to practice over and over and over and over and over until you get something that other people agree isn't too terribly awful (I bringin other people here because you'll lose objectivity pretty quickly during the process).
I've been doing this for years and my results are, well, uninspired. But we often have folks going through here who go from beginner to accomplished in not much time at all!