If you do not know what your parents think of gays and trans folk, then ease into asking them. "I have a friend that just came out to me. They want to be the opposite sex, and plan on getting surgery done, etc...". See how your parents respond to that, it should give you an insight to whether or not they would accept you. I said something along the lines of having a gay friend, and having a trans friend, just to see how my Mum would react, then i got into a conversation by saying i would also go on a date with someone that was, and it went rather well....