How do I tell my best friend I'm bisexual

Just tell her. If she's a true friend, she won't care what you're sexual orientation is.

bạn should stay calm don't worry about it. my best friend already knew that I was bisexual and me and her had a sleepover.

That's the same age I came out as bisexual. Don't feel as if bạn must tell her that you're bisexual. If bạn want to tell her, bring up something about LGBT members. Then, if she's cool with the topic bạn shouldn't have to be stressed to tell her. If she doesn't like LGBT members, she isn't worth your time.