Check that the water heater has power or gas supply
For an electric water heater, turn on the circuit breaker
For a gas water heater, open the gas valve
Set the thermostat to the desired temperature
Make sure the tank is full of water before turning it on
Open a hot water faucet to let air escape
Light the pilot light if the unit requires manual ignition
Wait for the water to heat up
Check for leaks or unusual sounds
Verify that hot water is flowing properly