Krita is really great! On windows though it's a bit slow under certain conditions, though it's been very actively developed lately and should hopefully see some speed boot under Windows. If you use Linux by any chance it's supposed to be much faster under that OS, I would try it but can't get my cheapo pen tablet working win Linux. There is Paint tool SAI, which is really quite nice under windows, though if I could I would probably use Krita. I actually got a free download of SAI from someone on DA, but if you really want to walk the plank matey, erm, I mean get it for free you should be able to find it(or PS) if you use good judgement for what you download.
As for photoshop, it's great for a lot of things, and a lot of very amazing artists use it for amazing things. However I personally think it's not the best tool for digital painting because it has no brush stabilization, something which both SAI and Krita have, along with great brush engines. It's also very expensive too, so unless you plan on doing lot's of photomanipulation and such.... Actually no, just start with GIMP, and Krita, both are free and open source, and quite good. For digital painting you can apply most photoshop tutorials to [insert painting program here] pretty easily. I think of you want to learn more about digital painting crtl+paint is a great website to check out! Also if you don't have a pen tablet, you should get one, and not a cheap crappy one like I did, real Wacoms start at like $50, so it's not a very big investment, just use all the money you saved not buying photoshop!
edit: looks like I'm a bit late here