You aren't pregnant now are you? If you are not pregnant yet, you need to wait until you are older to have a baby. A baby is FOREVER, and really ties you down. The mothers can tell you more about that though, as I'm 31 years old and I plan on never having children.
As for if its natural for a girl, I don't think I'm the best person to answer that, as I've never wanted children. Ever. Many women do want children, however, and it is natural for grown women to want children.
For right now though, just have fun being a teenager. You have the next 60 years or so to be responsible and grown up, don't rush it.