How do I replace a damaged valve on a water heater?
It should be a standard threaded valve (NPT, National Pipe Tread), most likely 1/2" in diameter. If you are not sure, you can do one of the following:


Lookup the valve in the manual, search the make and model on line if you don't have a hard copy of it.
Remove the valve and take it to the store with you.


Two would be the simplest.

When draining the tank, remember to open the hot side of a fixture or two as this is will allow air to enter the system and let the tank drain faster. You can keep these fixtures open until you refill the tank as they will allow the air to leave the system when filling the tank.