You sound like me when I was single. No idea on how to approach a guy you like. 🙂
I don’t think that the first time has to be necessarily flirty. Perhaps the second or third time. Just say hi to him whenever you run into him, ask him how he is and see if whatever his replies are will lead on to another conversation. Try to see if there’s something in common you can talk about – favorite sports teams, or things about the club you two are in. I think that’s a start. Show him you’re interested but don’t make it so that he’ll run off or something. Start something small. When he’s with his friends, include his friends.
If you want to be flirty after the initial conversation, you can always tease him. I used to do that to guys I liked. The important thing is to be yourself.
Hope it helps.