Where to go from here? I need help.

I've finally admitted to myself, after years of suspicions and cover-ups and just plain ignoring it, that I am transgendered. Now I don't know what to do. The only person who knows is my twin sister. Right now I'm terrified of telling anyone else, and I'm afraid to destroy the feminine persona I've built up-- it's been my protection for years and years, and the idea of showing my true self to the world makes me feel vulnerable. I'll probably start by talking to my therapist, but if anyone could give me an idea of where to go from here, where to begin, I'd greatly appreciate it.