I have a reliance 501 water heater, the pilot light won't light. What do i do?

1) Shut the breaker off to it at the panel. 2) Hook a hose up to it (the bottom has a drain). 3) Close the valve to the water heater (on the cold water supply). 4) Open drain on the bottom of water heater. 5) Open the relief valve to allow air into the tank (it won't drain otherwise - think if you put your finger on one end of a straw - the liquid stays in until you let go). Why are you draining it by the way?

The Reliance 606 has an enclosed thermal couple. Before you try taking that assembly apart take a wire brush to the bottom opening directly under the burner. There is a metal filter on the very bottom of the water heater. It could be that the filter has gotten filled with dust or animal hair. It took a serviceman 2 days to figure this out for me. The water heater has to have good airflow through that filter to work well.

Check the breaker and top thermostat and make sure they are not tripped. Move onto replacing elements if above is not the problem.
Turn water and power off to heater and attach a water hose to bottom of heater to drain down tank. May have to open hot side on a facuet to get it to drain.
Once drained remove elements with 1 1/2" found at local hardware stores. once elements removed and new ones in place. Turn water on and get the air out of the system. Once water is flowing from the hot side from all of facuet's in house.
Once all air is out now yaou are resady to turn power onto heater.