Well, if your mom loves you and knows you well already, then there is probably not too much to worry about. When I came out to my own mother this week, I slipped her a piece of paper at bedtime with "Transgender" written in large, pink letters, and said to not talk to anyone (including me) about it the rest of the night, to just think about it. It seems to have worked well for me, as my mother is trying to keep her negativity inside and tell me positive things, like there is no reason why I should need to suffer. My first therapist suggested the approach of keeping it simple at first, because anything after the word "Transgender" can become a blur to a parent in a coming-out letter. Definitely feel the waters before trying anything too deep. Caution never hurts, but please for your own future, get on HRT ASAP. I could have been so beautiful if I had started at your age; please do not let that opportunity slip away, as Testosterone poisoning only does more damage over time.