How do you ground a 120 volt outlet with only one white and one black wire?

The neutral is the ground

The neutral is NOT the ground. The ground is defined by a third bare or green wire in some cases. Look in the back of the receptacles junction box. See if the ground wire is bonded to the box but was not long enough to bring it out to the receptacle. With a tester you can check to see if the box is grounded. Place the testers red probe on to the black wire and the black probe on to the electrical box if it is a metallic box. If a voltage is noted, then the box is grounded. When the receptacle is reinstalled the ground will be picked up through the mounting screws. Or you can extend the ground wire at the back of the box and connect it to the green ground screw on the receptacle.

If you have any doubts about an answer that you get, check the answerer's bio by clicking on their name to check their qualifications.