For a free graphics software, you can't go wrong with the Gimp. It is a free program that is as good as Photoshop (better in some ways and not so good in others) but it is a bit intimidating. If you do decide to go that route, I would suggest following some of the tutorials in the Tutorial section of the site. I would start with a tutorial that works on the area, and in a style that suits, of the area that you are working on at the time...for example, if you are working on generating the land masses, look for a tut that deals with that, and when you are working on the mountains, look for one that treats that subject. One thing though, if you are planning on working your map from the start and don't want to "waste" time working on something else (and trust me on this, there is no such thing as wasted time when learning to map) keep in mind that most tuts either generate previous steps (land masses and rivers) or assume that previous steps are done. If you have already reached a point in your map that a tut goes over, just use what you already have instead. Read the whole tut though because you never know when something new might be useful.