It won't automatically fill. You must make sure that your water heater bypass (if you have one) is set to the normal position. Then you should open a hot water faucet and wait for water to run in a steady stream from that faucet. To be absolutely sure that your heater is full, lift the pressure relief valve located at the top of the heater to make sure some water sprays out. You should periodically check this relief valve anyway so this will accomplish that as well as ensuring that the tank is full. After all this you can safely turn on your water heater.