I know it's been posted here before -- the Prop Builder's Molding & Casting Handbook is a great reference. Don't limit yourself to one medium. Clay is great for some things. Wood is great for others. Sculpey's neat. And green stuff is awesome for small projects. You can also buy it in bulk -- just search for "epoxy putty." Also, if you don't already draw, or do some other kind of art, I'd suggest working on building those skills. What you learn with drawing will benefit you in sculpting, and vice versa. Getting your hands to do what your brain wants them to do is harder than you think. Oh yeah -- be prepared to spend a lot of money on mistakes. That's how you learn.