How do I wire a hot water heater?

Assuming a standard US water heater in a residence, which requires 240V/30 amps.

A double 30 amp breaker, 10/2 with ground Romex cable.

In circuit breaker panel, connect conductors (black and white wires) to breakers and ground to neutral/ground buss bar.

If the circuit breaker panel is with in sight of the water heater, connect the cable directly to the water heater. Connect the conductors to the water heater leads, connect ground to ground screw.

If circuit breaker panel is not with in sight of the water heater, install a 30 amp disconnect. That can be either a 30 amp receptacle and plug or a 30 amp disconnect box.

It also could be 120 volts/20 amp or 240 volts/20 amps. If it requires 30 amps, you need #10. For 20 amps, use 10 or 12.
A #6 will clamp onto cold water in, and at panel ground.