what is a good starting bike for a 15 year old male?

[anthony]

I am 15 and approx. 5'10.I really want to start racing motocross but im not sure what is a good starting bike. I was thinking about racing a yz 125 2 stroke. Also, how do i get good? do i just practice riding my bike in my backyard and in my woods everyday??? I do have some experience because ive had a Mx 100 for about 2 years. All my friends say i cant do it but im determined. Please help me