Happy birthday!!! :D
Don't worry, I felt just like you round about that age...I'm now 20 and STILL trying to find myself. All you can do is learn about yourself as a person right now...what are your likes/dislikes, what are you interested in, etc....
I think it takes people a lifetime of discovery and even then you might die without truly knowing yourself completely as you grow and change constantly as a person.
So just do what makes you happy :D Have lots of fun experiences, have regrets, do what you want when you want to just because you feel like it, smile just because you feel like it etc....Just be a teenager whilst you still can be :) Speaking from experience, over thinking situations and...just things in general can only lead to unhappiness :) So why bother? Just be a free spirit :D