How do I wire a hot water heater that is set up for 240 to 110? I don't want to have to put in a 220 plug I just want to run it off the 110 source already there. Any help would be great.

Check the amperage of the water heater. Most require a 30 amp circuit which will require 10 gauge wire. If your 110 source is 10 gauge wire, you can take your neutral off the neutral bar in the panel, and make it up on a 2 pole 30 amp breaker along with the hot from the 110 circuit.

If you only have 12 gauge wire and the water heater is pulling more than 16 amps.. you will have to run a new line to meet the requirements of the water heater.

How do I wire a hot water heater that is set up for 240 to 110? I don't want to have to put in a 220 plug I just want to run it off the 110 source already there. Any help would be great.
You need to check bldg codes. I would recoment you wire it per manufacturer. If you don't, make sure your homeowners insurance is up to date and paid.

You also need to replace the 240 volt heat elements to 120 volt. What you propose to do is not practical.

Summary of above. It would be safer and more efficient to convert wires to 240.
Neutral would need to be identified with black tape, Both ends.
You don't have t put in 220 volt plug, Hardwire.
Make sure nothing else on that pair of wires.
You might make a 120 volt recpticle 240 without knowing it, on same pair.