The switch at the back of the tank is for the electric part of the heater.On means your running the electric element. Off is off. You should also have a circuit breaker for the electric side of the water heater at the main power panel. The remote switch on the inside is for propane usage. Turn it to on and the gas section should light if all is well. Sometimes these atwoods get cranky and don't want to light. Make sure all the contacts are clean, the spark gap is correct and that the flue is free from blockage. If you google troubleshooting atwood, you should get all the info you will ever need