You need to see a doctor to find out why. There can be many reasons in both the man and the woman. Doctors recommend that if you have tried for a 12-18 months without anything you should seek help.
Yes there have been women who have taken fertility drugs and have had a great many children in their womb. Most of the children inside of the womb died because they were too small to live.
It is unusual but it has been known to happen. Most girls at that age have not yet developed the reproductive system to a point where pregnancy is possible. However some are early bloomers.
Surgical sterilisation has a very high success rate, but is not 100%, so if you are still fertile it is possible (although unlikely) that you could get pregnant.
Unfortunately, yes. "Pulling out" is never an effective method of birth control because sperm can be transferred before ejaculation. If you are sexually active, you should use condoms to protect...