These are the two main graphics editing programs that I know about. If there are others, feel free to tell me or post them in replies.
Personally, I prefer GIMP for many reasons. First, it is a free program, which, at 14, is very necessary. Although I did have an illegal version of photoshop, I removed it because I knew it was wrong, and I really didn't use it anyways. GIMP is the better program in my mind anyways. Although the open time is very long, especially if you have brushes (more especially if you got bored and downloaded over 2000 brushes from Devianart,) GIMP is more accessible. Not only is it quick and easy to get used to, there are dozens of available tutorials.
Please, post here if you would like more information, or just to pick one over the other, but please include reasons to refrain from being a spam post.