Mac computers are great for graphic design (and for everything else really except games, but there's gameconsoles for that, and you can even install windows on your macbook just for games.) I do all my digital photo editing on my 17" powerbook.
Photoshop is the best tool, it can do the most things. But it's pretty expensive. There are some cheaper versions around.
The free alternative is GIMP, it can do almost everything photoshop can do (except really professional print stuff) but it works very different compared to photoshop. Most image editing guides on the internet assume you are using the newest version of photoshop.
Other digital editing (free or not) programs are not as useful. You can try out things like arcsoft photostudio if you happen to get it free with a scanner or something, but I wouldn't recommend buying it (get photoshop instead.)
iPhoto and other photo programs can be useful too, but not for the real editing, just for rotating and fixing the colours and things like that. Aperture is very expensive, iPhoto is usually fine, and it's much simpler than photoshop. Some digital cameras also come with free photo editing software.