Pick your the friend relative you are closest to and you are almost certain will support your decisions and back you up come hell or high water. Then slowly start from there.
Personally I chose one of my female friends (had to tell her twice but that's another story). Then came out to more of my friends slowly one after he commented about my dress and that I wasn't taking female hormones (my reply yet).
My siblings I've yet to be able to tell due to time and location.
When I told my mother this time around she flipped out, placed a heap of restrictions on me on who and when I can tell (still debating what

shoulder I should listen to

). And accused me of this and that but she was told years ago and she is a bit of a narcissistic so.... Haven't and won't tell my father but I don't speak to him even when we lived under the same roof. So don't think you have to tell everyone.
I told everyone face to face because that's the way I'm most comfortable with so choose a method that you are comfortable with because you are probably going to be nervous enough as it is.
Hope this is helpful for you.
Gypsy