My now 30 year old daughter figured it out on her own when she was 16 or so - a normal kid's curiosity, looking in closets and dresser drawers. She had a conversation with Mom (we are happy married), who promptly pointed her to me. It really about discussion and being available to answer questions. A couple of things you need to be aware of - the Internet and her need to share a secret so she can talk about it.
She is no doubt searching the web. She is going to see the full breadth of our community, from respectable sites with great information (like here) to sites that cater to those who objectify and/or belittle us. I'm sure you've done this already, but make sure you tell her that your available for ANY question that may arise. Let her explore on her own, as she probably is anyway.
She now carries your secret and it MAY be a very heavy burden depending on her socialization. The thing about this generation that so amazes me is their broader acceptance of gender variance. She may be one of those people who have experience with people who just don't fit into a box and "I'm good with it". However, she still does share a secret with you. In my daughter's case, she "fell in love" with a real jerk'. It ended after a couple of years, as he was an was a terribly controlling and emotional abusive person. At 16-17, we opted to let this run it's course with out being overly judgmental, yet watching for signs of physical abuse. She figured it out and in the process of dumping him, he started to stalk her including coming directly to our house. At that point I confronted him and essentially threatened legal interdiction should he persist. He attempted to nuke me by confronting me about my crossdressing. The moral of the story is while you may ask that she not disclose your secret, she may feel the need to due to her need to talk to others that she trusts. Not much you can do about this. BTW, my daughter, my wife and I have the closest of relationships and openly have discussions about my gender issues. In hindsight, I'm so very glad she knows.
Best of luck.
Joanna