Question concerning Ocella birth control?

I recently got Ocella birth control pills, which is generic for Yasmin. I need to know when to start taking them and then how long I have to be taking them until they will be effective. I read the little leaflet but it was kind of hard to understand and i want to be sure.. normally I would ask my mom but she doesn't know i'm going to be ually active (she think's I'm taking the pills for cramps).