Bruce is right of course, just to clarify what he is saying. You need to measure 0 - 12V and the input on the ADC measures 0 - 5V (assuming you are using 5V & 0V as your reference voltages). You must never exceed the maximum input voltage of the PIC which I presume in this case is 5V. If your resistor chain had both the same value resistor, then the output voltage would be 0 - 6V. I chose resistor values that would roughly turn a 0 - 12V signal into a 0 - 5V signal. To make the equation simpler to understand I would have suggested a 7K resistor (you are trying to drop the output voltage by 7V) instead of a 6K8 and a 5K resistor instead of a 4K7, but these values are not available, thats why I suggested the values I did, hope this helps.