It depends on people and contexts, but generally, you can just tell them there's something about yourself you'd like to share, and then proceed to describe your feelings, and how you came to this conclusion. And of course, you can make sure you're there to answer any question they might have about it, it's important to give people a chance to learn more and maybe overcome the internalized prejudice that someone might have.
The most important thing is being comfortable with yourself and your sexuality. Figure out whether or not you’re ready to come out as bisexual. If you are, you should choose someone trustworthy and supportive to tell. Keep the conversation honest and positive, and you’ll be likely to have a constructive conversation.