I have a momentary switch that is usually on and I want my circuit to be off when the switch is letting the signal through, how would I do this?
Here's a typical low-side switch configuration:
You need to pick a N-MOSFET that can switch completely when Vgs is equal to your power supply voltage. If your supply voltage is very low, this might be difficult to find.
You also need the FET to withstand the full power supply voltage between drain and source when the channel is not conducting.