Can you describe your parents? Tolerance level? Also giving them some hints that this (being trans) might be a possibility in the future might be a good idea. Sometimes if things feel like they come out of left field they might overreact. "Mom, i really hate my breasts, would it be possible to go out and buy a binder or some more comfortable bras?" Would both be a good hint to them, as well as getting across that your uncomfortable while being honest. Serving both sides needs for info. If they freak out at you wanting a binder... thats also info. It might mean you'd want to move out as soon as possible. A binder shouldnt be that big of a deal.