Alot of people seem to like GIMP, but if you like a more traditional finish, Artrage is a great program which is very close to traditional media. Gimp I believe is a free version of Photoshop. No sure, but that's what I've heard.
GIMP is not related to Photoshop; however, they both do much the same tasks and have similar tools. I use PS Creative Suite, but GIMP is an admirable substitute for those on a budget. If you find you enjoy digital work you can always try a free trial or Adobe Elements, which is quite reasonably priced.
i have a bamboo fun thing too, my came with the full version (i think? nothing is missing) of photo shop and something else, I prefer photo shop because that's all i've ever used and i'm too lazy/scared to try to understand naother because of my learning difficulties (i just can't pay attention, i just have the 'IT'S TOO HARD' mentality)
you might want to practice with your tablet other wise your lines will be a bit squiggly like mine were, and layers are your friend :D