you should not test before your period, considering the accuracy of the test isn't that great, they say you can test up to 6 days before your period, but you should test with your first morning pee once you have missed the first day of your period, and test again 3 days later if the first test was negative for the best results.
I got a neg on 12 dpo 7 days b4 missed period waited a wek later 19 dpo got a pos
Everytime I got a BFP it was before my period was due but I know some woman who dont get a positive HPT until its late as their HCG isnt high enough. I would retest on the day AF is due with a First Response. Good luck!