Like Fiery_Nightmare said, paper, and MS paint are good ways to go. Also Photoshop is a program that is well known to artists. It's not completly free, but you can get a 30 try out from here
http://www.softpedia.com/get/Multimedia/Graphic/Graphic-Editors/Adobe-PhotoShop-Trial.shtml
Mind you though, if you plan on buying it, the program is very expensive D:
There is also the Gimp, which is 100% free http://www.gimp.org/
Hope that helps somewhat :3