Welcome to the Guild CorranHorn (one of my favorite Star Wars characters BTW)! Firs of all, you don't need any computer software do make maps. There are a handful of artists here who work almost exclusively with hand drawn media. However if you wish to learn how to create maps digitally, and want to know which software to get, then check out the link that Chashio posted. A good starting program is GIMP. It's free, fairly powerful, and well supported in the tutorials sections here. You may also wish to browse through the tutorials sections to see what styles are possible given different forms of software.

Cheers,
-Arsheesh