If it is a girl you are really interested in your mind might be filled with doubts and insecurities :
What if she says no ?
Am I trying to move too fast ?
Maybe she thought we were just friends and she will be offended if I ask.
All of these fears, although natural and common, are dumb. If you want a date with this girl you have the right to ask her out. Don't allow your life to be ruled by doubt and insecurity.