I have a simple example of the basic code side on
this page about a quarter of the way down, labeled
Analog Input example.
But what you're looking for is probably the
PID Example close to the bottom.
That demonstrates the use of a steering potentiometer to position a motor based on the position of a joystick.
For your fixed position arm you'd replace the whole joystick input with just a constant = the exact voltage you'd want the arm potentiometer to read when the arm is where in a fixed position.
You'd have to add a case statement to chose the different values you want the pot to have.