If you want to ride motor cross, you need a two stroke to learn. Since your size is kind of small, I would find a Kawasaki KX 65. Fast little two strokes. Once you learn and grow get (any brand) 85. My little brother has a 2007 Honda CR 85 one hes 14 and 5' 1" and he rips on it, it also has enough power for me 6'0" and 200 lbs and it pulls wheelies in 2nd gear no problem. Also remember 2 strokes are always going to be cheaper and you should get a cheap bike for your first.