Any repairs done to a Moen faucet require the water supply to be shut off. Failure to do so could result in flooding or injury.
You will first need to determine where the faucet shutoffs are located. Generally, they are located under the sink, in a closet access behind a shower or in an access panel around the tub. There are some cases in which the shutoffs will be your main home shut off (This will turn the water off to the whole home, usually found around where your home water meter is located; basement, outside wall, garage, crawl space or close to the street). Rotate the shut offs clockwise until they are tightly closed. Turn the faucet/shower handle(s) to the open position to insure the water is completely off.