Step number one
Before a girl can like you she has to know you exist. This step assumes that the girl doesn't yet know you exist. Maybe it's a girl you know in school you've seen her around you think she's really pretty and you like her but you don't know what to do or what to say. It is very simple next time you see your walk past her in a casual pace, turn to her, and then just say hi. At this point she's either going to really notice you or she's not.
Step number two
In this step we are going to test to see if when you walk pasted the girl you like and said hi if she really noticed you. A day or two later place yourself in close proximity with the girl somehow. Stand there and look at her and wait for her to look at you. See if she smiles and says hi. If she does then you definitely made an impression the day before and she probably likes you. At this point you simply walk over introduce yourself and shake her hand. Engage in a brief conversation with or maybe three or four minutes. The idea here is you want to make yourself no longer a stranger but you still want to be a mystery.
Step number three
Today when you see the girl since you are no longer a total stranger you are going to approach her, walk right up to her, say hi and ask her out. Nothing too complex or deep date wise. Maybe grab a burger at a burger joint, get coffee, something simple casual and uncomplicated.