Okay, so two of my friends know I like girls, but they think that I ONLY like girls. How do I say that I'm bi without just randomly saying it? Like, what would be good opening?
You could comment that a guy is hot. That would be an easy start. "You only like girls!" "No, I'm bisexual, I thought you knew that?" That's really the only way I can think of doing it, that doesn't involve awkwardly blurting it out. Or, next time someone suggests a girl to you or calls you a lesbian or anything like that, just reply "You know I like both girls AND guys, right?" Point is, be reflexive, and reactive, if blurting it out on your own is awkward. This sounds exactly like a meme I saw once
Wait for someone else to bring up the topic by talking about some "hot" boy, and then agree with them. If they say something like "I thought you just like girls" you can then add "I like some girls, but I like some hot guys too", without using any technical labels such as bisexual, pansexual, or lesbian. You are still quite young; you can wait until your specific preferences solidify before giving yourself a hard label.