As long as your water heater is not bypassed (winterized) it should automatically fill up when you hook up to city water. To help it along you might want to turn on the hot water taps on your bathroom and kitchen sinks when you first hook up to city water.
Once you start getting a steady flow of water coming out it will be safe to actually turn on the hot water heater. You do not want to turn on the hot water heater when it is not full as it will cause damage to the water heater.
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