First, Welcome to Susan's, Ruki!
I am sorry that you are stuck in that kind of situation. I know you feel that you need to tell them, but given your situation, I would hold off. I would first stop wearing the dresses and makeup, instead of going all the way. That would at least free you from the worst of the femaleness (in terms of what you can change, obviously it's not worst of body parts and stuff like that). Granted, you'd still have to wear feminine clothes, but at least it would be jeans and t-shirts instead of dresses and skirts.
Also, if you decide to tell them, I would find another family member/friend to stay with if things go badly. I would also suggest talking to your school councilor if you have one(so your parents don't have to know, because some parents are nosy).
I wish you the best of luck, and I hope that your parents accept their son. Let us all know how it goes.