Look under the sink. Two water lines extend from out of the wall up to the underside of the faucet. The hot water line should be on the side of the hot water faucet handle. Two separate lines also feed the washing machine, and may feature a valve or a lever.
Twist the hot water valve clockwise until it stops turning to shut it off. If the hot water line uses a lever, push it in the opposite direction from its current position. For instance, if the lever is currently upright, pull it all the way down to shut off the water flow.
To shut off a water heater for temporary work, twist the cold water supply valve above the heater clockwise until it stops turning.