What kind of dirtbike should I get?

I have a Honda trx 300ex four-wheeler and i know to ride it really good. But I just want to switch up and get a dirtbike. I'm 15 y/o and I'm a boy and I'm about 5'9 or 5'10 and I weigh about 150. Now I'm not going to be riding in any trails at all. I wanna ride motocross and I want a good mx bike that will fit me I was thinking about a rm 85L or a crf 150r or maybe even a yz 125. This us gonna be my first dirtbike so I could use any advice thanks.