Hm 15 is mid teens I'd say! Still, if you're leaning more towards the same sex, then I can understand if you're secure about your sexuality . Being open at first is tough, but... you'll find that once you've told your friends, and you can actually just come out and say 'yes, I'm gay' to just about anybody, it's such a relief. You can go out to parties or town and not feel pressured. You can put forward your views without fear of 'suspicion'.
Don't feel pressured to just blurt it out though. We all do it in our own way.