Go with a Crf250r, Honda four strokes are bullet proof. You're a little old too start racing. I started racing when I turned 5. But if you get a bike, and Practice every day, and Put your mind set too only motocross. Get a good workout routine going. Get too the track atleast 2 times a week, or build a track in your backyard. I used to race in the 450a class at my locals and sometimes out of state. Now I do enduro races and desert races. Competed in the Baja 500 last year. If this is what you really want talk to your parents seriously, No joking around.
Good luck, Wyatt
MMI mechanic, Been in the motocross business for 17 years.