Its going to be attached to the throttle body. When you step on the gas, there is a cable that goes fro the pedal up to the throttle body and pull the throttle plate open. The throttle body is sprint loaded to close it when you let off the pedal. Look for that and you'll find the throttle position sensor attached to it along side. It will have two wires coming off it and probably only held on by two little screws.
It is how the computer knows how much throttle you're giving it. I don't know the process by which it determines fuel flow rate, but it is logical to assume the throttle is a critical component in that calculation. If it is acting stupid, it may very well throw your fuel mixture off which will adversely effect mileage in the end among other things.