Awww, I'm so happy for you! I wish you the best in doing this...it's good to prepare something to say beforehand. The most important thing you can do is keep a level head during the conversation! Speak from the heart, and be genuine. Doing it in person is better, I think. You're their child. They'll be more apt to listen to you if you look them in the eye when you tell them what you are.
Also though, you should consider how your parents might react. If it seems possible that they might not be accepting, I hate to say this but you might want to consider waiting a little while. If they aren't accepting, you could find yourself in a really, really bad position. You seem very young, and being kicked out really wouldn't be a good thing. Do you have a contingincy plan in case things go badly? Don't plan on having to use it, but you should have one.
Best of luck,
S
