It's a difficult situation- This is going to sound maybe like a stereotypical response, but I mean it with all my heart.
I told my mother back in January, in my case, I had already made an appointment at the Center for Sexual health here to see a psychologist and well, go over everything. I'm nearly 23 so... The responsibility there lies on my end. After I made the appointment, I went out to her one night, and I just opened up with the fact of the matter "Mom, we need to talk..." the conversation lead on a few lines, stating that it was something serious, and that mattered and she needed to listen (she... never really listens to me much,) anyways... After that, I just came out and told the truth "Mom, you know I've never fit in with any guys I know, and I've known for quite some time that I need to be female, and I want to emphasize the word need, not want... I've made what I think is the proper decision and booked an appointment to see a psychologist specializing in this matter on Feb 19th."
It was the hardest paragraph I've ever spoke, I choked up and stuttered a lot... But I said it. My mother I knew wouldn't be accepting, and she wasn't, and she's still not. I'm not trying to tell you you have to tell parents/family now... But at some point it will happen, and in the case of myself, my ex (whom was MtF,) and my friends.. It was all that was possible.
You should definitely steer the conversation if you can, with my mother though... I had to be blunt, as I knew (and was correct) she would react negatively instantly, after I said it... The conversation was done for the night, just her making accusations, telling me it was a phase, etc...
I wish you luck though, if you need to chat ever... Just let me know!