When I was in my teens my father found out about my need to be a girl. Suffice to say it did not go over well at all. He had me parade around the house in front of him my stepmother, and my stepsisters dressed up in my sisters' clothes. He asked if I wanted to be dressed that way all the time, and even though in my heart I was screaming yes, I replied in the negative. I was shamed into hiding at that point and I strove to suppress myself up to this past year. That was 24 years ago. Just this past month I finally mustered the courage to tell him that I was tired of living the lie that is my life, and that I am going to transition. That has been the hardest step for me yet because of my past experience. However, it went really well. He accepted it and welcomed me as his daughter instead of his son. And he even apologized for what happened in the past.
Now, from what I have seen from other people on a general basis, it is a major shock at first. But after a some time most families will come around. Just remember it is your feelings that you should worry about. Don't live your life for what everyone else wants. You will not be happy. I personally recommend telling them in person. It is very difficult to start believe me. The way I told my family was "there's something I need to tell you. I don't know how you will handle it, and I apologize for any pain it may cause. But this is something that I need to do for myself." And then I told them.
I don't know if this will help you. I hope it will. Good luck, and keep your head held high.
Shawna