Well, What you have to do, Are these steps:
1. Find out what they feel about LGBT via non-direct questions.
2. Find out what they would react when you say "What If I was LGBT?"
3. Catch them on a good (but not too good) time to tell them. (Mostly Saturday, Probably, When they're relaxed and not busy)
4. Be truthful and let them know that you will still love them just as much as before.
5. Let them know that not everyone are alike, And this is the way the makes you happy.
6. If it works, Have a wonderful life.