Locate the ball valve along the gas line that feeds into your water heater. The gas line is usually colored yellow and the ball valve will likely be red or blue. Use a flashlight to see in dark areas.
Note the position of the ball valve. If the valve is turned to the fully "On" position, there is nothing more you can do to adjust the gas pressure. If the valve is not turned all the way on, but hot water is being produced, turn the valve all the way to the "On" position.
If the valve is partially in the "On" position but there is no hot water, turn the valve to the "Off" position and wait five minutes. Turn the valve to the "On" position, then turn the gas valve on the hot water heater to the "Pilot" position. Press down on the "Ignition" button until you see the pilot flame inside the heater. Turn the gas valve to the "Run" or "On" position. The gas is now fully on.