Pinball - School project

I´m making a pinball machine where the user has controll over the two pieces that you use to "move" the ball. You will be able to move these freely within a set area, but if you move them outside of this area, there will be a sound and lights go off and you loose. What I need help with is figuring out what sensors to use and how to code the sensors so that they will operate within the given area.

user has controll over the two pieces that you use to "move" the ball

"Flippers"?

What I need help with is figuring out what sensors to use

If you want to sense a pinball ball, i.e. a shiny metal ball (like a ball-bearing), how about using beams of infra-red light? The ball could break the beam and trigger a signal on one of the Arduino's digital inputs. You will need IR LEDs (permanently on) and IR phototransistors (connected to the Arduino). You'll find quite a few threads on the forum that go into ways to connect IR and visible light sensors to digital inputs.