In order to unlock your device, you need only to type in the provided fields the Google email address/username and password that were used to configure the phone. If you don't remember the username, you can recover it using the I can't access my account page. If you have lost the password, you can recover it using Forgot your password?. Alternatively, you can unlock your phone performing a factory reset. Turn the device off, press Volume Up + Power buttons and wait for the Recovery menu to appear. After that, select Wipe data/Factory Reset and follow the instructions.