To add to Alain's explanation, GP3/MCLR is an input only. But huh... why using POT while your PIC have an ADC converter? Check in your manual and on this forum for ADCIN.

Make sure your pot impedance meet the datasheet requirement (<2.2K if my memory serves me well)

If you're using a Photoresistor, you will need to use a buffer (op-Amp, transistor or else) in between to match the impedance.