get a quality BMX bike, hit the local dirt jumps/skate parks/street spots, get some bike handling skills and THEN get a bike for shredding trails on. i'm so tired of seeing kids on overbuilt bikes that they can't handle flinging themselves down the trail, getting bucked because their bikes are designed for much bigger people, trying to emulate guys twice their age (and size) who have the strength and experience and know how to handle themselves and their bike.