So how do I set the pressure in the tank?

With the tank empty, you set the air pressure to 2 pounds less than the cut on pressure of the pump switch.

Harold is likely correct but you have not given us much. Is this the water tank on a well system?

By "tank empty", he means pressure drained to zero by opening a low level faucet with the power turned off. Pressure in the tank is set by means of adding/bleeding air from the air valve at the top of the tank.