Test the water. if you see something in the news about gay marriage etc, make it into a bit of a discussion. See how they react to the whole notion of homosexuality. If they react well, come out. If they seem quite uncomfortable, think about. Will they see past the sexuality? One would hope so. If they shout and scream about immorality, keep it under covers until you leave home.
Only you know your family's opinions on the matter.
But at 17 I would hope they would not dictate who you see, even if they disagree.
But I would come out if at all possible. My aunt did to the family when she was 18. Everyone was brilliant about it.