I take it your water heater doesn't have an electric element for heating....gas only and you want to 'convert' it to electric heat.
After market elements kits are available (Hott rod & Lightning Rod)
They screw into the drain hole (1/2" on Atwood Brand and 3/4" on Suburban Brand), have their own t-stat and plug into an outlet. You just have to provide the outlet.
Suburban Brand requires the use of an anode rod....the kits come with a small rod.
And BOTH MFG. do not recommend using a drain hole heating element and will void warranty if used.
Now IF your water heater came with electric heating element....those are std.