Information adapted from Fiske Countdown to College.
Don't pick the teacher who gave you an easy "A." Instead, go for the ones who know you best, or for whom you worked hardest, or for whom you did your best work.
Check the requirements of your colleges before you ask; you may only need one. Be sure to apprise the teachers of your application deadline.
Fiske Countdown to College has more tips and information about letters of recommendation, including when to ask, how to follow up, and whether it's okay to ask a teacher who doesn't teach a core subject.