Is giving birth painful for a 13-year-old girl who desperately wants a water and natural birth?

Using water in your labour, and having a 'natural' birth will greatly increase your chances of having a rewarding, positive and less painful birth. Having a water birth will help lessen your chances of tearing, it will help relax you (which means you aren't fighting the contractions, which means they're less painful) and having it naturally means you aren't being tampered with, which allows your body to get on with doing what it was built for: giving birth.