Most Helpful Girl
Well, if sex is all you want, you have to go for a girl that is into casual sex/hookups, otherwise you're boned.
Step one: Find a girl who is open to casual sex/hookups.
Step two: Get to know her enough that she at least finds you charming.
Step three: Present that you want to have casual sex/a one-time hookup with her.
If she says no, be cool and respectful and tell her that's okay, then move on. If she says yes, bring a condom, find a safe private place, and do the do.