I was just wondering, if I want the sound to be killed when I press down on the switch, do I need a normally open or a normally closed switch?

I'm guessing a normally closed switch because that means the circuit is closed when its left in its "normal" position, and then when its pressed down, the circruit is opened.

Quote by ljplum12
Normally open, wired between the output and ground.

Will this give the undesired result of having it give a"oops I unplugged my guitar" kind of sound?
Nope, using a normally closed to simply "break" the signal going to the jack will have an unplugged lead humming amp effect. Wiring a normally open between the two lugs on the jack will have a completely quiet effect.
These go to eleven...