There are three leading programs to choose from, Adobe Photoshop, Adobe Fireworks and GIMP. If you haven't heard of either Fireworks or GIMP, you're not alone. Fireworks was a Macromedia product, and since Macromedia was acquired by Adobe, Fireworks is part of Adobe. GIMP is a free, open-source program which is surprisingly powerful, and has almost all of the features of Photoshop.

You should download GIMP, and see if you like it. There are books available on using GIMP, as well as online tutorials; find one that you like, and see what you can do. You may also want to check out GimpShop, an add-on to Gimp to make it look and act more like Photoshop.