You would normally use the supply voltage and ground as the positive and negative voltage references, and they would also be the ends of the potentiometer. The wiper of the potentiometer is should be connected to the analog input of the 16F88. A capacitor from the wiper to ground is a good idea.
The code needs to enable the correct input, wait for a few microseconds (the acquisition time) and then start the conversion. After the conversion time, the "Done" bit is set and the A/D result can be read from ADRESH and ADRESL