There are so many materials usable, suggesting may not be that easy, kiddo! If you want to start small - say, under 1 foot - you may choose a plastic clay which never hardens. Such allows large/small changes at any time whereas if your material hardens, you're locked in. However, non-hardening clays are not suitable for long term keeping if you like the outcome... Unless you go the moulding route too. I suggest you try one of both first... WallyMart sells two kinds of cheap white clay which handles well & air-drys. Online, there are various clays available. Stay away from the bake-to-harden materials... I've found them problematical for anything larger than jewelry. You might also think about getting some armature wire to support soft materials.