How do I tell my friends I'm an introvert?

There is nothing wrong with that at all. I am that way too. I only had 1 or 2 close friends in high school and that was it. Maybe its because your a bit more mature than your friends. It was weird because with me, I was always more comfortable and talkative around adults than other kids my age. Thats actually a good thing. You don't need a lot of friends to be happy. Just be who you are, and if they can't accept that move on and find some friends who are like you. Trust me if you are in high school it may seem like the most important thing in the world right now, but it will mean nothing after you graduate. When your in the adult world things will change for the good for you :)