It depends what you want to do. For example, if you want to start off, then use a pen and paper. That's a great way to sketch out a map. Take a photo, or scan your sketch, and post the picture here. Then we can take it from there.

Gamerprinter makes a good point with Gimp and RobA's tutorial. As you've not mentioned what kind of map you want to make, that's a good place to start.