How do I read the output from a guitar patch cable?
No, you don't need to supply any power to the guitar, but the signal from it is very low level (a few millivolts RMS at most). You'll need an amplifier to bring this up to a level that your ADC can actually read (a volt or two RMS) &mdash; a voltage gain of about 1000&times;.

Assuming you have a typical guitar amplifier available, try reading the values coming out of the headphone jack using your ADC. You haven't said what kind of ADC you're using, but if it's the typical one found on most microcontrollers, you might need to add some DC bias to get a complete waveform.