I wish I could tell you an easy answer, but there isn't one. You just have to do it.
I know it's hard. It took me months to work up to it. The conversation, when it happened, went something like this:
Me: There's something I have to tell you, and it is hard to talk about. I don't know how you'll take it. (Letting her know that we aren't going to be talking about the weather)
Her: Oh?
Me: I am pretty sure I am transgender.
Her: Why do you think that?
Me: Well, all my life, I have wished I was a woman.
Her: What are you going to do?
Me: I don't know yet. I know that I don't want to leave you. (Important to get that in early)
Her: Whatever you do, I will support you. (Me, to myself: YAY!!!)
Obviously there are no quarantees that your conversation will go like that. Before it happened, I had already decided that I would accept any outcome. The only outcome that was unacceptable would have been to say nothing. I had rehearsed what I wanted to say many times in my mind. I had almost started to say it many times, and had chickened out. When the words finally came out, it was like I was somewhere else, listening to myself speaking.
Good luck! You can do it.