Once you have some allies, or at least one. If you have a sympathetic therapist, they will help you with coming out strategies for parents as well as classmates and your school itself.
If your parents are supportive and have the means, you can get on puberty blockers. Depending on where you live you can get on HRT by age 16, but again, only with your parents approval.
Read and educate yourself on what is all involved, the problems you may have and what you need to do to follow your goals, in short, make a plan and know why your doing it. Just like planning for college when in high school.