You simply need a voltage divider to scale the battery voltage to whatever the PIC is running. If it's on 3.3V (for example), then taking roughly 1/4 of the battery voltage would suffice (although a nominal
12V battery may well be somewhat above that).A typical solution (to account for a battery of up to 14V) might be a divider of 33K and 10K (33K connects to battery, 10K connects to 33K and other end of 10K to ground, take the junction point to the A-D input).
This yields a division ration of 4.3:1 for ADC readings (i.e. 12V on the battery will give 2.79V at the ADC input).
Simply scale the data from the ADC accordingly.
All PIC ADC inputs have cautionary notes on their input impedance. In this case, the input impedance would be roughly 7.7k from the divider pair, assuming a small output impednace from the battery. You could put a small cap (0.1uF) across the 10k, but this will increase the sample switch required time. If you aren't worried about incredibly fast conversions, that's no real issue.
Cheers
PeteS