You should know your best friend well enough to judge what way would be best. Maybe coming right out with it in a serious way would be good, or maybe you want to first claim you're bi in a joke and see how he/she reacts. Maybe it would be best to ask and find out what your best friend thinks about bisexuality first.
In the end it's your best friend though, and if you being bi is a problem to him/her, then obviously it wasn't really a best friend.
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Well because you are best friends he probably may like you, and do nicer things.
He will have an erection if he likes you.
I'd ask him, ok? He's your best friend ok? If he says no he really doesn't want you to know, or he isn't. If he says yes, don't hate him, he may act different now that you know, but don't hate him ok? Its never good to hate someone cause something major kinda happened. I know one thing for me happened, my best friend was bi, and it turned me bi.... although I was already starting to think I was.
You can tell your best friend by choosing a time when you both can have a private and calm conversation. Be honest and open about your feelings, and let them know that you trust and value their support. Remember, your sexual orientation is a personal and important aspect of yourself, so it's okay to share it with those you trust and care about.