If you were 100% sure of what you wanted, then the sooner you start taking control of your life, the better.
However, that's a VERY big "if." I'm not saying it isn't possible to know what you want at your age, but the odds are against it. I've noticed several of the others here have told you to go slowly and seek some professional guidance before you do anything drastic, such as starting hormone therapy or coming out of the closet. For most trannies your age, I think that's very good advice.
If I knew then what I know now, I would've tried to find a way to start taking female hormones before puberty set in and started pumping testosterone all over the place. Also, I would've spent my college years and early employment years as a woman, which would have made things easier on me in my thirties, because now I've got a lot of social considerations to deal with if I ever want to make the transition to full-time. I envy those who were able to start their transitions early in life.
It's very rare to be that young and so sure of both yourself and your environment that you could decide to transition early without any reservations. If you think you are one of the few who can make this decision now, the first thing to do is tell your parents, unless you've got your own money. Your parents will need to be involved in getting you a good therapist who can help you be sure of your decision and help you move forward. Not only do most trannies your age need financial support, but your family might be able to offer you the emotional support you'll need while going through the transition. Being a teenager is difficult enough without be a transsexual. Sometimes we just have to admit to ourselves that we need others to help us.
I was unable to tell my mother about myself until this year. I didn't think I could afford to do that until I was independent, but even after that, I continued to stay in the closet out of habit. The pressure kept building, though, and I couldn't continue to keep everyone from ever getting too close to me, and I figured family was the best place to start. When I was in my teens, I was in no position to make a transition. I was comfortable with myself, but there were just too many things about the world I didn't know, and I wasn't in any position to act on my desires, even if I had been sure about what to do. It's a shame that youth is wasted on the young, because that's the perfect time to transition, but it's the worst time to be certain of your decision.
Proceed with caution.
Having told my Mom and my doctor recently, I'll say this:
If you know someone isn't tolerant, don't risk it. However, if you think someone is tolerant enough, go ahead and come out. Awkwardness is a tough thing to deal with at the time, but once it's done, and the other person has accepted you, it's more than worth the temporary strangeness you feel when you initially tell someone.
