mrjonny2:
I would use a linear regulator to drop that voltage down to 5V to prevent any damage to the arduino and then have the potentiometer to give your 0-5V range
Glad you are not on my project. Only a beginner would ever consider doing this. It is just so wrong.
Yep. If you want a 5V supply you wouldn't start with a 12V battery, you'd use the Arduino 5V pin(!)
OP: You don't need a potentiometer. A 1.5k and 1k resistor will do the trick.