If your not using birth control because you are sexually active, can you still take it wrong?

I talked to my doctor about taking birth control just in case down the line I decided to become sexually active, and for my period. But I have very irregular periods so I don't know when I should start taking it. I just ended my period I guess you can say around three days ago, but I just receive the pills today. I was wonder should I start taking the pills now or wait. Since I'm not sexually active even if I start it wrong after the first month would i be on track.