Although it is true that Adobe Photoshop is the best art program out there, it is also the most expensive. It generaly costs several hundred USD. If you are completely serious about your computer art (like it's your job or area of study), it is well worth the expense. However, if CG art is mostly a passtime/hobby, I would suggest getting a cheaper alternative.
A much cheaper alternative is Corel Paint Shop Pro X. I happen to own it and I am pleased with it's performance. PSP costs less than 100USD. If you wish to see an example of my art done on PSP just click
here and
here. It may not be the best CG out there, but for casual use, PSP is definitely a good choice.
(i also did my avatar and siggy with PSP)