What is the best dirt bike brand to start out with if you're 13?

depends on how much you weigh and how tall you are. But from an average 13 year old i think a perfect fit is a TTR 125 L or the LE, only difference is the LE is electric start, and the L stands for big wheel because they also make a 125 for about and 8 year olds size. If your a little bit taller and heavier go with a Kawasaki KLX 140 or a Honda CRF 150.