There are really no magic words. Just be open and honest. I suggest just jumping right in with "I'm gay." Then the rest of the conversation will be directed by their response, so no big lead up to the announcement is really needed. I know that coming out to family is scarier that coming out to friends, but there's no difference, really, in that you cannot control (or even predict) other people's reactions. It will be what it will be and nothing you do will change it. So, just be yourself and it will be ok.