Bald spot- 17 year old girl?

Hi, I'm currently a 17 year old girl going on 18. It seems that I have a thinning patch of hair on the front of my head... going bald. How can I stop/cure my hair from receding? It looks very unpleasant at the moment. What may be some causes? I'm a vegetarian, but I ingest sufficient amounts of vitamin C everyday... what may be some other factors? Thank you.