Ditto what AVid said.
The valve that your looking for should be located in a short (about 12") line between the cold water line going into your hot water tank and the hot water line coming out.
These lines may be color coded, blue for cold and red for hot. The valve should be open when it is parallel with the line and closed when perpendicular. At least mine is set up this way and possible your's will be too.
Hope this helps.