The advice both Labman and Speedball offered is correct, thou I have a bit more to offer.
NEVER turn on power to an electric water heater UNTIL the tank is completely full of water, under pressure and ready to go. Check with the plumber before energizing.
If your the plumber, turn on the water, let the tank fill, bleed all air from tank and pipes, check for leaks, turn off water and remove all pressure, fix all your leaks, and start the whole process over again.
If your like me, you will do this about 3 or 4 times till you get the damn water to stop leaking.
Most electric water heaters need a 30 amp 240 volt circuit, using a 30 amp 2 pole breaker and #10 2 wire cable with ground. Check the wattage and voltage of the unit. A 30 amp 240 volt circuit can handle up to 5760 watts.
And what usually is missed, and is required, if the water tank is not near or in site of the breaker panel, then a two pole 30 amp rated disconnect switch is required at the water heater. This is to have a switch close if the tank needs to be serviced.
Plumbers (should) appreciate a disconnect switch (and demand) at all appliances that have electricity, so they can easily shut off the power to work on the unit.
Oh Tom, we never rough wire the water heaters first, plans are never correct, or the plumber changes his mind and moves it, so no water heater gets wired until it is all plumbed. Once the plumbing is connected, we know that it will not move, since plumbers hate change orders.