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.

You 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 you must tell her that you're bisexual. If you want to tell her, bring up something about lgbt members. Then, if she's cool with the topic you shouldn't have to be stressed to tell her. If she doesn't like lgbt members, she isn't worth your time.