First of all, you should already be interested in the usual forms of art such as drawing and painting, and have some skill in that matter. Doing Graphics using photoshop have NO artistic skill required, and even if you rock at Photoshop, you could suck at pixeling. Try starting off by reading and performing all of these tutorials:
www.pixelart.com.
They're very good for starting off, as they outline the basics of pixeling, and some lighting basics as well.
I'm not a good pixeler at all, but those are my favorite tuts :-)
Good luck!