My advice first off would be avoid the term "bisexual" until you're sure. I'm not sure if this is as common as it used to be, but there's sometimes a trend of people (particularly younger girls) claiming to be bi just to be "cool" or get attention from guys. Please don't misunderstand, I am not saying this is what you're doing at all. I just want you to be able to confide in people and them take you seriously. Maybe describe it as questioning who you like? Now, if you're sure you're bisexual, it's ok to say it. Just be prepared for alot of misunderstanding and the possibility of having to explain that this is for real.

Do any of your friends have LGBT friends or family, specifically that they seem accepting of? If they do it would definitely be best to talk to one of those friends. If not, try to get some idea of how accepting they are of gay or bi people. Bring up a gay/bi celebrity, mutual friend, etc. and try to figure out their views from that. Good luck.