Uhhhhhh, no dude. You just go up and start dancing with them, with a (genuine, not forced) smile on your face so you don't look intimidating. If you are genuinely having a great fucking time, girls will dance with you. If you are not having a great time, it's probably not a good idea to try to dance with girls on that particular outing.
If you tap girls on the shoulder and ask to dance, you are probably coming off like a creep(I mean, you could pull this shit off if you were the sauvest motherfucker alive, but that goes with anything). If you try to dance with a girl and she's not having any of it, who cares, theres a bunch of other girls in the place that might want to. If there are no/few other girls in the place, go to a different party.