You have to enter the credentials for the account you've previously used in order to gain access to your phone. Google and Samsung added new security features in order to prevent theft and illegal use of stolen or lost devices. Although this is not the case, you still have to add the Google account to your phone to proceed.
If there is the possibility you've lost access to your Google account, follow the next link to learn how to reset it: https://support.google.com/accounts/answer/41078?hl=en
