[Mike Swearingen]

Shutting off the cold water supply to the water heater shuts off the hot water coming out of it. Turn OFF the power or gas to the water heater first.
Open the highest hot water faucet in your house for air and then the drain at the bottom of the heater tank (attach hose). The pressure and the water in the line need to be drained down. You don't need to drain the whole tank, but you can't solder with water in the line.
Yes, you can use a small wad of white bread (no crust) to temporarily plug the water draining out of the line in order to solder it. I have done it many times. When you turn the water pressure back on, remove the sink faucet aerator to flush the bread out through the spout.
Another alternative, since the shut-off valve is exposed under the sink is to simply use a compression fitting valve (no soldering).
Good luck!
Mike

 

[VIPlumber]

Now would also be a good time to install a stop and drain valve on your hot water line, somewhere near your HWT. That way, the next time you need to do any work on a hot water line, you can shut it off and drain it at the same spot without having to shut down your HWT.