How to approach girls in college?

Long story short, I'm an average looking 18 year old college student, and I've been single my whole life. With my class schedule, and most other people I assume, I don't see the same girls very often. That makes it really difficult to form any sort of friendship. I don't have the confidence or looks to just go up to girls I don't know. The only way I would be able to do that is if I was sure she was attracted to me. So my question(s) is what are the "signals" that I should look for? How could I tell if a girl is into me if I'm in class with them? And how should I approach them?
How to approach girls in college?
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