It's as LM said. I'll add: don't turn it on unless the water heater is full of water. Probably a brief on/off blip won't hurt. Since you don't have a manual (?) I should also mention that there's a water heater bypass built into the water lines, so when you want to fill and use it you'll have to close the valve on the bypass and open the two valves on the water heater's in and out lines. Once the water heater is full you just turn on that switch and it's automatic from there on. You'll need to have the gas on too, the heater is gas only. Ours is very quiet--you have to listen close to hear it operate. It's very nice to have hot water from the tap.
The dealer should include the water heater operation including the bypass location in the final walk through. You might have some luck contacting FR and asking to get an owners manual. Anyone else here have a number to call at FR? I vaguely remember a posting about someone contacting FR customer service about an A-frame issue and getting a positive outcome.
Open the heater's outside access door and ensure that the anode rod has been installed before trying to fill the heater. The anode rods are often left uninstalled--and rattling around inside the access door--when the heater is not filled. We leave ours out between camping trips so the heater will dry inside (we put the water system back in bypass mode then, of course).
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator