Wrong section, but what the hell:
If there the nicer kind of adult, go straight out and tell them, sit them down, preferably when there in a good mood, and tell them how you feel, then hope for the best. If there not so open to this kind of gender related thing, then maybe a letter? Slide it under there bedroom door, send it via email while your at work/school/out or maybe get a school counseler, or therapist if you have one to help you get your feelings about what you want to be/are through to them. Hopefully they will be understanding. Best of luck, justsomekid1.