Only you can decide... you don't have to tell them. If and when has to be your decision, no one else's. mx:))
Your sexuality is really none of their concern. Look at it this way, would you be torn about it if it were a guy you were having sex with?
depends on how close you are to them and how relevant they are in your life, if you live with them then obviously it'd simplify things but if its going to end with someone getting hurt then its not necessarily their business
in a situation like that i'd say wait untill you're independent or moved out to university/own place untill you talk to them about it, just for your own well-being
that'll change up your entire priority list then, your parents and "old life" takes a back seat to everything going on directly around you, i'd say wait untill then at least, see how much you feel the need to tell them once you're settled in at college, then if it still feels really important, sit down and consider it properly now you aren't totally dependent on them, new friends will help you make the decision too :) that's my personal opinion obviously but i hope its somewhat helpful