How can I make friends in high school?

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It's already been a month and I only made 3 friends so far (besides the others I already have) and then some other people are just people I say hi to. There's like a group of girls I see at lunch and some of the girls are in my classes, how can I become friends with them? I say hi to a few of them and I talked to them on facebook once. How can I get a friendship with people? I don't know what to talk about at school and I don't have alot in class to talk to them. Should I talk to them on facebook more and then try to get there numbers so I can text them and so on?