Check the gas valve leading to the hot water heater. Make sure it is in the "On" position where the handle is parallel to the gas line. Look underneath the water heater to ensure the pilot light is turned on. If not, continue troubleshooting.
Turn the gas control valve on the side of the unit to ensure it is turned to the "On" position, as indicated on the handle. If so, continue troubleshooting.
Turn off the gas to the unit by turning the valve to the "Off" position, perpendicular to the gas line. Look for a sticker near the pilot light that indicates how to adjust the pilot light valve flame. Locate the instructions in your owner's manual if there isn't a sticker. Turn the screw on the pilot light valve according to the instructions to adjust the pilot light flame. Turn the gas back on and turn up the thermostat. If the pilot light doesn't turn on, continue troubleshooting.
Turn off the gas to the unit. Loosen the bolts holding the gas line and the thermocouple line onto the gas control valve. Pull out on the lines to remove the burner assembly from below the unit. Rub the thermocouple with an emery cloth then reassemble the unit. Turn on the gas, turn up the thermostat and check the pilot light. If it doesn't turn on, replace the thermocouple.