You can start birth control anytime, but when you start it on your period, you are protecte dimmediately. This should not matter though, since you only need to take the pill for 7 days for it to be effective. That should leave some time for it to be effective before you get to see your man.
You can start now, but you must use it 7 days for it to be effective. Aka no unprotected sex for 7 days.
You may still get some light bleeding when your period is due, you may get nothing, each woman is different