Obviously, choose a private indoor place, not a busy public one. Or, a tranquil open-air park or beach without too many people is nice. When you break the news, be positive. She'll immediately tense up and brace for the worst. It's better to be more casual, indicating that you've got something meaningful to talk about.
Negative words send out signals of shame.
Positive words mean unique and special. This is a lot to handle in one sitting. If you start throwing out biological and sociological theories, or your transgender life story, it's going to confuse her. Keep your words simple and to the point, and don't use our communities' slang and acronyms. Reiterate that you respect her enough to feel you have to be honest with her, and that you're not gay and not transsexual.