Birth control question

Alicia • 21• Michigan

On the 3rd I went to the obgyn. It was to get a checkup and to get on birth control. I decided on the pill. I have read the papers it came with but I’m a little confused. When she did the chlamydia exam (with the q tip) she said it looked like I was ovulating but my app said I wasn’t supposed to be ovulating until the 12th officially but the ovulation window would be the 7th-13th. My last period was early so I can only assume this one would be. I’m pretty sure my periods starting but it’s REALLY light right now. My doctor said to take it the Sunday after the first day of my period

When should I start taking it