Irregular period affecting birth control?

On oct. 11th i had to take the plan b pill and the womens clinic gave me oral birth control pills. they didnt explin to me how to take them. So finally i read the the paper in the box. and it said start the sunday of your period. by the time i had read this my period had already came. And i have very iregular periods so there not dependable. so how do i know when to take them