I have a GE Performance Plus Refrigerator (side by side with the freezer on the left and a separate door (for milk etc) on the front right). Anyway, it is leaking terribly and I need to turn off the water to avoid any further damage. Does anyone know how to completely turn off the water so that it doesn't leak any more.

See if there is a valve under the sink, or find source of that line.

You will have to trace the line back to where it starts. Pull out unit from wall and you may find the valve there. I would guess that the main problem along with the leak is that it is dirty. While you have it out from wall unplug it and remove rear cover at bottom of unit and remove front kick-plate. Slide out drain pan and clean in sink with soap and bleach. You should be able to see the coils with a good flashlight. Spray down coils with a good spray soap then wash out with sprayer on water hose, then vacuum up water with wet-vac. Clean fan blades and oil motor if it has oil holes. Put everything back together before plugging it back in. The rear cover must be on (make sure any vent on cover is clean) for air to be routed through coils. After you get it clean you will be able to better see where any leak is located or you may not have one.

Here a manual that may help you find and fix a leak: http://dl.dropbox.com/u/8010163/GE_Side_by_Side_refig.pdf
You still should clean it first.