How can I tell my best friend and boyfriend that I'm bisexual? Is there certain things I should say or not say? Please help.

You can simply tell them about how you feel, what it means to be bi, how you realized you are bi. You can encourage to ask you questions if there's something that isn't clear to them, and you can also talk the prejudices on bisexuals, and why they aren't true - I know a lot of people misunderstand the bi reality because of those. If they care for you, they will understand that nothing changes in the person you are, you're always you, and they will accept you and love you for who you are.