How does an analog input pin work?
The potentiometer and the internal resistance form a voltage divider. The voltage created by this voltage divider is essentially indistinguishable from 5V regardless of what the pot is set to because the internal resistance is so high.

What you want to do is to create an external voltage divider that will cause the voltage to vary full scale between 5V and 0V. You do so by connecting one end of the pot to 5V and the other end to 0V, and drawing the voltage from the wiper.