Look at the drain valve at the bottom of the tank. If this is leaking call a plumber to replace the faulty drain valve.
If the leak is coming from the tank you will need to replace the water heater.
For electric water heaters, find the water heater’s circuit breaker and flip it to “off.”
For gas water heaters, look on your water heater for an on/off switch or dial. Turn it to “off.”