Just be yourself.
If on the dancefloor, Walk up, tap on the shoulder and speak in the ear (if it's a loud environment).
If outside (in the "smoking area"), speak as normal, and ask a straight-forward question, eg: "Hey, I'm new in this town & I'm in my first year at uni (make something up). Are you at uni too?"
If not in a uni town, and you're just going clubbing in general, just ask any question that comes to mind. Maybe pick something that the guy is wearing like his shirt or a particular ring / bracelet or shoe style and talk about it. Ask why he wore that particular item today in the club.