Got the pill for the first time???

A

I just got the pill from my doctors today and I'm kinda confused when to start it because she said start on the day your period is meant to come but why

However my cousin says I should start tomorrow so my body can get use to it and whatever

Please when should I start it any ideas

And my bf and I are planning to have sex on Monday so I obviously won't to be ready for that

Im just lost