I'd love to make it available to purchase as a poster. Unfortunately, the company I was working with previously to sell this as a shirt had apparently received a cease and desist letter (presumably from someone at Cartoon Network), so I don't want to push anyone's legal buttons by selling this design.
Check out Paint.NET. It's powerful, Photoshop-like software, and it's completely free and fairly easy to use. They have a fantastic, friendly, helpful community at their forums, filled with tutorials and examples of other people's work. They should help get you on your feet.
Well...I've done all of my work on a PC or Mac, so I don't have much experience with using a tablet for this sort of thing. If you're going to primarily be using the Nook for your work, I would suggest becoming proficient with pen and paper first, and possibly getting a capacitive stylus.
Otherwise, try going the computer and mouse route. Of course, I'm bein broad with my suggestions. I don't know what your particular artistic interests are