If you start taking the pills now, they will do nothing for your previous sex.

They will not harm any pregnancy that you could very well have right now.
They will not prevent that pregnancy from happening.

They will prevent any pregnancy after one week of proper use, but any unprotected sex that you've already had is not going to be affected. So basically, if you think you might be pregnant, you really should wait until your period and test. You could take these pills starting now, but that's going to make your "cycle" be 6 weeks long (since it was 2 weeks from your last period, and the pills are for a month).

Personally, I'd wait the two weeks you have left until your next period, see if it comes, and test if it doesn't. Then start the pills if you're not pregnant. Follow the instructions on the pills. Because you will be able to start the pills on the first day of your period (if it comes) then you will be protected immediately from pregnancy.

Use a condom until then! Condoms are very effective at preventing pregnancy.