It all depends on how well you know the girl, i know you can just straight up ask some girls...but most you need to get to know some through situations or just through friends, if you like her and you want to get to know her and get her number, invite some of your friends and her friends and ask them to bring her, go mini golfing or something watch a movie at your place an have some games to play or then go mini golfing, bowling, whatever you like to do. (i am asuming you would like her number to date her) and just be yourself around her. :) hope things work out for you.

