I highly recommend GIMP. It's free and open source. My viewpoint on GIMP is that it has/will have every feature other paid programs like Photoshop have, except you'll have to wait a little longer. A great trade-off in my opinion, since it doesn't cost anything.
I hear Paint Tool Sai is also good, and it's much cheaper than Photoshop (I believe Sai is only around forty dollars, but I may be wrong), but it only works on PCs, and I have a mac so I don't have firsthand experience.
If I were you, I would download some free programs, and possibly free trials of paid programs, and test them out. Figure out which one you like best, and if it's a paid program, try to find a free/cheaper program with similar features. For example, GIMP recently introduced a Smooth Stroke option, similar to Sai's Stabilizer. Best of luck!