How can I attach an analog to digital converter to a guitar
The output impedance of a guitar is in the order of 10k ohms. If your op amp circuit has an input impedance of say 100k ohms, there will be no significant loading of the guitar output nor any affect on downstream amps or effects boxes.

In order to keep the input impedance of your added circuit as high as possible, pick off the signal with a unity gain buffer and then follow that with a gain stage as needed to match your uP ADC input. An LM358 would be a suitable op amp, for example.

Do take care to keep your op amp input leads short to avoid stray pickup due to the high input impedance.