Resonator or crystal? Ceramic resonators are three pin devices and have built in load capacitors, so none would be needed, accuracy is less than a crystal though. For a crystal you should chose a crystal that was designed for the same load capacitance as your PIC, (which is easy) most common crystals that operate on fundamental frequencies use 18-22pf capacitors. You can use trimmer capacitors to fine tune the frequency if needed, though if you need trimmer caps for frequency it's probably a good idea to oven control the entire oscilator section. (Pic and crystal) Or use an external oven controlled oscilator.