It's a fairly broad question, so I guess here's a fairly broad answer...
There are several ways to approach this, depending on (among other things) coding skills, and budget.
Cheapest option would be to use something like OpenCV, and a webcam: you could use a feature tracker, or a colour (blob) tracker to get x/y positions, and use that to change anything on screen. You probably already have all the hardware needed, and an example of getting started with colour tracking is included in oF: check the
A bit more involved would be using a Kinect to track the body of the person in front of the wall.
examples/addons/kinectExample is a good starting point.