Since you mention 'from a pedal' I'm going to assume that you are trying to connect a guitar or keyboard expression pedal to your Teensy. Also, as you mentioned being a beginner, it may be useful to read up on pots from that perspective:
Resistors and Potentiometers used in Expression Pedals
All About Expression Pedals
So, you probably have the wiper from the pot (the centre terminal) connected to your Teensy. The first thing to check is that you also connected the ground from the pot (the end not connected to positive voltage) to ground on Teensy. Otherwise, you will just get random readings. Voltage is a difference in electical potential, and it is measured relative to a fixed point, usually ground. Also, check you are reading from the analog pin you are actually connected to

(this sounds silly, but I have done it myself and again you just get random readings).
If that is all good then your readings should at least be changing as you move the pot.
Next thing to check is your power and how clean it is. 5V from USB connected to a computer has a lot of digital noise on it. Capacitors can help with that, as mentioned earlier in the thread. Using battery power, which is common in pedals, can also help. 3 AA batteries will give you a clean +4.5V. Or a PP3, which is often found in guitar pedals, will give you +9V which is too much for Teensy. But you can still use it - take the wiper output via two resistors (say, 10k - the value is not critical as long as they are both the same) to ground, then connect the point where the two resistors join to your Teensy. This is a voltage divider, the maximum +9V is now halved to a maximum of +4.5V.
Using a screened cable will, as mentioned earlier, help avoid hum pickup. If you are using battery power then a simple tip-screen (two-pole, 'mono') guitar jack cable will be fine as a connection. If you are using +5V from Teensy then a tip-ring-screen (three-pole, 'stereo') jack cable with two core sheilded wire will be better.