@ammna,
If you are talking about the regular birth control pill that is prescribed to be taken daily, it takes about 2 weeks to be effective. So every day at approximately the same time of day, you are supposed to swallow a pill. After two weeks you are 99.9% likely to avoid a pregnancy.
But when you got the prescription, the doctor should have told you all this and the detail instructions are written on the package.
Why the heck would you take a medicine and not know the details of how it works?