Start off by connecting your web cam to processing, there are libraries that will do this. Then work out how to detect movement from the web cam. You might want to look at frame subtraction, that is subtracting successive frames, no movement shows up as zero (or close to it). Then calculate the position. Only then do you need to send that position down the serial port to the arduino which just takes numbers from the serial port and drives the servo to that position.
I hope I get a good mark. 