My period is very irregular and I just got prescribed birth control. When should I start it?

I get my period at the weirdest times. sometimes weeks in between, sometimes months, and it lasts from 2 or 3 weeks, to a month or more (with light days in between, and then heavier). Anyways, I just got prescribed Yasmin, and my doctor said to take it when I get my period, but that can be months from now... So, will anything happen if I start it when I'm not on my period? I mean, my cycle is really messed up, so... would it really matter?