Trying to wire a water heater to a breaker box
Call an electrician, if you're asking these questions you don't have the basic knowledge required to complete this safely and properly. 

Without additional details it's literally impossible to provide guidance to you. There's many variables here such as if this is a standard main panel vs a sub panel and the type of water heater you're installing. You also didn't include the wiring gauge you're using or the input voltage from the utility (120V , 230V, 240V). 

Dangerous and improper wiring kills over 500 people every year in the US, don't be one of them.