Ok, most people use photoshop, but this costs a whole lot. Personally I have Photoshop CS3 EXtended, but probably any version is ok.For those who don't want to spend money, there is another option, GIMP (Gnu Image Manipulation Program) which offers many of the same features but is generally not as good. GIMP is freeware and can be gotten off of www.GIMP.org
You should start by following tutorials. There are a couple on D2JSP but also there are lots on deviantart.com, Blacksungfx.com (must register to read), tech-gfx.net (must register to read), or most other well established graphic design websites. You (of course) would want to start with beginner level tutorials and work your way up as you get to know your way around Photoshop/GIMP. Although most of the tutorials are for Photoshop, they can usually be changed to work in GIMP too with a little of common sense. Remember, you should LEARN something from a tutorial, not just follow it and move on. Learn and experiment with outcomes and START SIMPLE.
Hope this helped a little, PM me if you have any quesitons