You could get pregnant from having sex on that day, if the sex was unprotected. (I hope not, though. Why are you having sex so young, and having it unprotected? There are all sorts of perils to doing that, not just pregnancy.)
As the other poster said, unless you are still in the window to take Plan B (within 72 hours of the unprotected sex), all you can do is test, 2 or more weeks after the unprotected sex.
Take a pregnancy test at least 2 weeks after sex.