Well, to start with, pin RC3 is a digital I/O pin, so you're not going to get any voltage other than 0 or 5 volts out of it.
If you want to measure voltage, you need to use one of the analog input pins, which typically are on port A, and have names like AN0 and so on. Once you have the hardware plugged in to the correct pin, you will need to use the A/D converter to read the voltage, which is more involved than just assigning a variable - check your compiler manual...
- Jon