Like Axelle said, you would know best when to talk to her. However, nothing will change who you are, so you will eventually need to anyways if you want to maintain the relationship.
I told my best friend about two weeks ago. Was I scared? Absolutely. But, everything turned out very well, and we even had a more in-depth conversation about hormones etc. yesterday.
Just be sure that when you feel the time is right, to try to stay relaxed and be as straight forward as you can. Think about what you're going to say ahead of time, but try not to get wrapped up on possible responses.
Best of luck to you!
*edit*
And think of it this way, you have an opportunity here to start your life over. Sure, it can be scary, but what are you worrying about? What others think? It's a difficult thought, but now is the time to be selfish. You only live life once, and live it to the fullest. Not doing anything about being transgendered will only make matters worse for you in the future. For me, I've pitfalled numerous times because of the gender dysphoria. I know my parents would rather see me living a happy successful life, as myself, compared to a failed miserable one where I try to please everyone else.