apple login and password

I am using a phone Iphone 5, Model: A1428. FCC ID: BCG-E2599A. IC: 579C-E2599A.IMEI: 013429002724280. I buy someone else's product, when I reinstall Iphone 5 asked to enter the account username password to boot the machine. I do not remember your username and password. now my computer is locked, I want to use the phone.

please help guide me. I sincerely thank