As sweetpea03 says, it could be stress, but it's most likely hereditary. Ask your parents and close relatives when their hair started to go grey. Going grey in your teens is unusual but perfectly natural.
There are also some conditions that can cause white hair, notably a deficiency of B12 or a thyroid imbalance, and if you're very concerned and don't have a family history of early greying, you might want to ask a doctor whether they can give you some information about these conditions or test for them.
Don't worry too much though! Going grey doesn't mean you're cursed to look old. You can easily buy cover-up dyes, and there are many very cool styles you can pull off with grey hair. My father knew a girl in high-school who went white-grey in her teens, and he says she was one of the most beautiful people he knew, because her hair was so interesting, ethereal and cool. :)
Stress can cause you to have some white hairs, or it may also be hereditary if your parents had white hairs at an early age.
Thank you sooo much! Yes i have asked my dad and apparently, it is an inherited thing :S. thanks again for that info. im currently doing research on everyhting so must add that to my list!