GIMP! Don't even bother looking at other programs pal :D
-Gimp is free also for commercial use thanks to public license
-Gimp is user friendly for beginners and professionals
-You can adjust the GIMP interface according your own likes
-Gimp can do also pixel art and very detailed paint work
Oh, and get a Wacom tablet. Never use a mouse in graphic softwares...
I had a genius tablet quite a bunch of times and those damn products kept breaking in 4 months. Wacom tablet lasts like, forever! I even got a really old one that still works after fixing its cable that has been abused by its previous owner :O
Did I mention GIMP?http://www.gimp.org/