Dirt Bike FAQ

Hi, I'm a 14, almost 15, year old. I weigh about 105 pounds and I'm 5' 7, but growing a lot. I don't know what size and kind of dirt bike I should get. I was thinking 100 but I'm growing, and I also need a good beginner bike because I have never ridin one in my life. I have no clue what bike to get, and I'm kinda broke. What should I get?