You will need to find out the amp load of the unit you want to hook up. Usually they are around 30amps. That rating is typical for a 40 gallon water heater.So if you find that yours is 30 amps you will need 10 gauge wire. 10-3 romex is generally what is used. You will hook the red and black wires to the red and black leads in the unit. The white wire will be used as the ground. YOu will need a 30 amp two pole breaker in your load center to hook the other end of your 10-3 romex to. Put red on one side and black on the other. Hook the white wire to the neutral or ground block in the panel.