Do you have any image editing software? If not, download GIMP and Inkscape to start with. They're both free and not too hard to learn to use.

Then the way I did it was just to scour the Tutorial section of the board here. I literally took an hour or so and flipped through the pages of Tutorial threads and bookmarked anything that looked interesting. Run yourself through a couple tutorials, picking beginner ones to start with and then working your way up. Your first attempts probably will not come out exactly the way you want, but stick with it and you'll see yourself starting to get better and better. (And able to work faster and faster.)

This one's great for getting the hang of basic battlemapping in GIMP:
NewbieDM Tutorial: The Battlemap, a guest post by Jonathan Roberts | www. Newbie DM .com