I'm 18 and had sex with my boyfriend of a year for the first time a few weeks ago. We used a condom but me being paranoid as I am decided to take plan b right afterward just in case. A few weeks have passed and I just received my period. It came right when it was supposed to. I got a pack of birth control from my doctor and it says I should take it on the first day of my period. Well I'm just wondering if it is okay to start birth control after my first period since plan b. (it really just seems like a normal period but I read a lot of stuff online about hormonally induced periods). So can I start my birth control now?

You can, or rather you should have when you started your period. Make sure to use a secondary form of birthcontrol, preferably barrier like a condom, until your periods stabilize on the birth control. It should only be a month but might be two.

Plan B is not effective days after. It can alter or even make you skip your period. But it has no bearing on starting other Birth Control. You needed to have started it when you did your period. (as directed)

If you missed this time, you will need to wait till next month to start it as the doctor instructed.

Also remember that no birth control is 100 percent, so it is best to use condoms even if on birth control. (and pregnancy is still possible even then)