Well you can use photoshop if you have it or you can use Gimp which is free. Gimp is basically the same as photoshop ( not sure, I've never used photo..)
get gimp from:
and for some great tutorials to follow for the Gimp:
I'm not familier with Photoshop. I know you can either buy it for heaps and I think you can download a trial version or sumthing...
umm well i hope that answers your questions
or! you can just use paint! I've seen some pretty good jobs done with paint