say hello, be nice, act interested in her, then after a few minutes say oh shit, i forgot i need to meet someone but can we have lunch sometime? and ask for her number. or something like that.
if you aren't ugly or obnoxious, you'll be surprised how successful you'll be.
anyway, the worst that will happen is she'll say no.
then you move on to the next girl.
actually, i move on to the next girl even if the first girl says yes