I am converting from gas hot h20 heater to electric, I need a wiring diagram in order to wire the electric heater into the breaker box.

I have asked to have this moved to electrical. There really isn't much to it. Usually hot water heaters only have the 240 volt heating element. Check the rating of the tank and select the next large rating of circuit breaker and wire. Run 2 conductor with ground. Use a double pole breaker with a contact for each leg of the box. Beware of the breakers meant to give 2 circuits from one leg. Connect ground to ground and the black and white wires to each contact on the breaker. At the tank, connect the black and white to each terminal and ground to ground. Mark the white at both ends with black paint or tape.

I'm going to kick this over to the electrical wizard But since the plumber both pipes and connects the power to a heater let me describe the process.
The electrician installs a 220 breaker in the box and runs a three wire lead over to where the plans show a water heater will set. We bring in the heater, pipe it and connect the three wires in the recessed panel on top of the heater to the lead from the breaker box using wire nuts. After we finish piping the unit we pressure up the heater to check our work and move on.
If I've missed something We'll let our resident electrical expert take it from there. Regards, Tom

The advice both Labman and Speedball offered is correct, thou I have a bit more to offer.

NEVER turn on power to an electric water heater UNTIL the tank is completely full of water, under pressure and ready to go. Check with the plumber before energizing.

If you're the plumber, turn on the water, let the tank fill, bleed all air from tank and pipes, check for leaks, turn off water and remove all pressure, fix all your leaks, and start the whole process over again.

If your like me, you will do this about 3 or 4 times till you get the damn water to stop leaking.

Most electric water heaters need a 30 amp 240 volt circuit, using a 30 amp 2 pole breaker and #10 2 wire cable with ground. Check the wattage and voltage of the unit. A 30 amp 240 volt circuit can handle up to 5760 watts.

And what usually is missed, and is required, if the water tank is not near or in site of the breaker panel, then a two pole 30 amp rated disconnect switch is required at the water heater. This is to have a switch close if the tank needs to be serviced.

Plumbers (should) appreciate a disconnect switch (and demand) at all appliances that have electricity, so they can easily shut off the power to work on the unit.

Oh Tom, we never rough wire the water heaters first, plans are never correct, or the plumber changes his mind and moves it, so no water heater gets wired until it is all plumbed. Once the plumbing is connected, we know that it will not move, since plumbers hate change orders.

"If your like me, you will do this about 3 or 4 times till you get the damn water to stop leaking". Reminds me of last night, I decided to refit my cabin with ball valves, looked in my box, saw a roll of teflon tape and went ahead. 58 Miles away from my main house, taped 3 of 4 fittings, 3/4 male CPVC to the valve and Oops,no more tape. Doped it with pipe dope, tightened it as far as I felt safe, cemented the CPVC and turned on the water. That 1 last fitting dripped about 3 per minute. So, for the time being, I have an aluminum foil trough that runs into a 5 gallon bucket. I'm catching up, on 2nd try now! Brian