How to make more friends?

At my school there are groups. People in my grade already have groups they are in, so it is really hard for someone to go in a current group they are in, into another group. I mean there are the so called "popular" group, then there are the "nerds", then the kind of popular, and so on. It is basically what you see in all the movies. But then there is me. I don't really fit in either the popular nor nerd group. So I don't know, I have a few really good friends, or atleast I think they are really good friends, but I don't know if they talk behind my back or actually hate me. And I am typically quiet in my classes, so I don't know how to make more friends and become more out going.