Birth Control

I'm going to be starting my birth control here soon, and I just wanted to know since everybody I asked couldn't give a straight answer.... When will it be active in me, to where if I do have sex it will be preventing from pregnancy... I told my boyfriend when it's active in me, that'd I wanted to have sex because we are both ready, but my doctor said I can't start it till my period comes.. And my period is late by 3 weeks... Again I know I'm not pregnant because I haven't done anything... Please help me?