Lots of good advice here. Yes, certainly there are logistical reasons to start as soon as you can with at least hormone therapy. That said, you can't start anything until you're ready and honestly IMHO you're not ready until you've reached the point where you've decided your own mental health is important enough to make it worth the potential struggles. Each of us finds that point at a different time. You've got a good start being that you're young and have already accepted what is going on (unlike me, for instance, who sat in denial for 39 years).
Transition is an uphill battle and you don't want to fire that first shot until you're sure you're ready to see it through no matter the costs. I'm not trying to tell you not to move forward, quite the opposite, but you need a certain mindset to be successful. If you're gonna do it, I want you to be successful. So that's where, as others have mentioned, a therapist or counselor is needed. Coming out to your parents may help you find one, but of course there is the risk of what will their reaction be. A school counselor may or may not be helpful, but might be worth a try.