If your water heater is leaking you should immediately turn off the heat source and the water supply. If your water heater is electric there should be a double breaker labeled water heater in your breaker panel. If its not labeled just turn off all the double 30 amp breakers. If your water heater is gas locate the gas thermostat located on the front of the hot water heater about a foot above the bottom of the water heater. You should be able to follow the flexible gas line (attached to the left side of the gas water heater thermostat) over to a valve located on the other end of the gas flex. Turn this valve to the right ¼ turn. Once you have the heat source turned off locate the cold-water inlet. This is usually found on the top of the water heater it will be the one on your right as you face the front of the hot water heater. Follow the pipe up from the water heater till you find a valve. It may have a wheel or a lever handle. Either kind you will turn it clock wise to close it. If it has a wheel handle it will take several turns if it is a lever handle it will only turn ¼ turn. Now that you have the water and the heat source turned off call a professional. A water heater should never be replaced by anyone other than a licensed plumber. At Benjamin Franklin Plumbing® of North Austinour trained professionals will make sure your new hot water heater is installed to code. They will also explore all your options when it comes to your new water heater giving you the opportunity to consider many of the new energy efficient options such as tankless.