How do you unlock a Samsung sgh-t819?

Usually you can look underneath the phone battery. Look for a small sticker with numbers on it. Look for the IMEI code. (Thats part of the total code)

With that code written down (CORRECTLY!) just google search how to unlock your phone model.

Lots of sites have the other part of the code. Just be sure to enter it correctly. Entering the incorrect code multiple times will permanently lock your phone. Then you have to go through customer support, and that's always a hassle.