Uhhh...Not sure if this is of much help, but...
Firstly, talk to her more often than you would to other gals;
expressing interest.
Second, as what Ellisbeck said, be sincere and true.
I think that's mostly it... But as Klishcia said, it also depends
on what kind of girl she is; more details. I myself have never been
picked up by any guy before, but there was a guy who wanted my
phone number; I freaked out. Another guy too, I think, and I think
I scared him away with the gaze I gave him. 
Most girls would give their phone numbers; I think I'm the oddity.
