Photoshop is the man - bit complicated if you're not 'puter savey but it sure does some wonderful stuff for photos. Tons of lessons/tutorials on line. 3d cafe is a good spot for info.
As previous posters have said, Photoshop is it. Probably too advanced for beginners to use. How much editing do you want to do? basic cropping, red eye removal? or more complicated editing?
Lots of cheaper (Paintshop Pro) and even freeware available that would do those things. like IrFanView, Polyview or Faststone Image View (v good for organising pics too and it's free)