How do you let a guy that you've never talked to but see all the time know he can approach you and how do you know if he is interested in the first place?

approach him. don't be afraid. if he likes girls and you present yourself in a good way. he will start talking to you. don't bore him or overpursue. just introduce yourself. let him know you've been meaning to talk to him. just give him time he'll come around. Simple..Smile when you see them.