Help!!! I’m 17

I’ve never had birth control and I still don’t know how it completely works and I DIDNT start birth control yet.

Backstory: my period is really messed up do to the fact that u am having sex so my period has been really long and sometimes continues and never really stops. So idk when my period is.

Current: I think I have my period kind of mid way in...I really think I have my period.

Question... can I start birth control mid time of my cycle? Or I have to try do it on the beginning of my cycle?