When should i take a pregnancy test?

i had unprotected yesterday. we used the with drawl method, when he pulls out before he comes, but that doesn't always work and i'm really afraid that i could be pregnant. how long do i have to wait to take a pregnancy test? i was thinking about two weeks...what do you think? my periods usually come at the beginning of the month, which means i've already had my period for this month, if that helps any.