Like they said, be sure the valves are in the correct position, turn the hot faucet on. It may take a few minutes to fill the tank and purge the air. When water is flowing through it shut off the faucet and turn the switch on. You said it has one switch so it's probably a DSI (direct spark ignition) gas only model. If it had 120 volt electric heating ability it would have another switch for that.
When you turn the switch on it will click several times then you will possibly hear a sort of roar as it fires up. It will depend on where it's located as to hearing it or not. It's normal.
Open your bath faucets too so as to purge all the air or else your pump will be erratic because of air in the system.