Photoshop is great, but ridiculously expensive. I think you can get it for a discount if you're a student, though. If you have a Mac, there's a free program called Gimp that I use mostly for picture editing, but it looks like it does a lot of the same things as Photoshop (though I used Photoshop for years but never tapped its full potential because with what I used it for, I just didn't need a lot of its functions). So for me, Gimp pretty much does everything that I used Photoshop for, and it's free.
I'm not a graphic designer, though. I just do it for fun, so there may be more software out there better suited for just graphic design.