Welcome to Android Central! If you've completely forgotten the unlock pattern and the phone doesn't prompt you for your Google account and password, then your only option is to do a hardware key factory reset from Recovery (
https://support.t-mobile.com/docs/DOC-28014), which also means the phone will be wiped. In addition, after the reset and before you can set the phone up again, Google's Factory Reset Protection feature will require you to enter your Google username/password before you can proceed.
If anyone suggests Android Device Manager, I don't think that will work, because you can only use ADM to remotely set a password if the device didn't have a lock set already. You might be able to use Samsung's Find My Mobile, though:
https://findmymobile.samsung.com/