There are two steps in shutting down an electric hot water tank. First you want to shut down the power. Never attempt any repair on an electric hot water heater without turning off the power. Even removing the inspection covers can be dangerous.
Turning Off a Water Heater Breaker
Turn off the power at the breaker panel. Don't be satisfied with just shutting it off. If you have lock out lugs for your breakers, you can put one on it.
Another option would be to tape it and mark the tape so that no one turns it back on. As a further precaution you can let everyone in the house no you are working on the electric. Tell them not to touch the service panel.
One final check. Use a voltage tester (a non contact voltage tester will work good) to make sure nothing is hot inside the covers when you open them up. It is not worth getting shocked or hurt when you are replacing a hot water heater element.
WARNING!! Make Sure the Power is OFF!!! BEFORE You Work on Electrical Devices!!!
Cooling the Hot Water
For many repairs, you are going to want to cool off the water before you attempt to work on the tank. With the power off, the water will no longer be heated, even when the temperature drops.
To cool the water temperature off, turn on a hot faucet in one of the sinks, do not add any cold water to it, let it run full hot. Let the water run until it feels luke warm. If the water coming out of is warm to the touch it will be safe for working on the tank.
A properly installed hot water tank should not have a shut off valve on the hot water outlet side. This will prevent the tank from building up too much internal pressure. Shutting off the cold supply, is all that is needed when working on your hot water tank.
If you have two valves above your water heater, you will need to determine which one is the cold supply line. As you face the front of the tank, it should be the valve on the right side. This may not be fool proof, since the extra valve should not be there.
Turn on the hot water in a faucet. You can tell by feeling the pipes, which one is the hot and cold. Tag the cold water valve so that you know which one it is to shut off. Wait until you have cooled the water down inside the tank to shut the water off.
Draining a Hot Water Tank
Some repair projects will require that the tank be completely or partially drained. This is a bit of a job in itself. See the article, 'Draining a Hot Water Heater', for information and instructions on how to empty the tank.