Piezo voltage through guitar vibration and Nyquist's theorem
Voltage will be low in the mv range, you should have a high impedance say 1Meg resistor in parallel with the piezo, and then feed that to a little opamp preamp.

As far as sampling, nyquist is the bare minimum, I'd probably sample much higher but give it a try and look at the waveform you get.  If low sample rate is enough for you then go for it.