gimp is great, there are plenty of tutorials out on the internet that'll help you learn to use it. Its not the most user friendly, but i found it to be really good for what you "pay" for it.
photoshop is the industry standard, but also costs quite a bit =\ However, the price is definitely worth it as i find it a lot more user friendly than gimp and, as a result, my workflow is a lot faster with it. Student price is a little under $200.
photoshop lightroom is what i primarily do everything on, its very intuitive, easy to use and pretty much all my photos go through it, even the ones that are eventually sent to photoshop for further edits. With the student discount, it comes down to a little under $100
aperture is pretty good imo, although i dont have much experience with it. i prefer lightroom but if i was forced to use it, i can definitely get aperture and gimp into my workflow.
if you're just starting out, you really shouldnt worry too much about editing. At this stage, worry more about getting a camera and learning basic technique and composition. As for processing, start out small. Basic stuff like exposure correction, color correction and whatnot, leave major digital plastic surgery for later
as i found out early on in my PP endeavors, less is more unless you're a pro, cause chances are, you dont know what you're doing.