See this page for information on color keying, including links to tutorials and articles that explain how to do it step by step:
There's nothing fundamentally different about doing this work with a movie as a background compared with doing it with a still image as a background.
If you're new to After Effects, I very strongly recommend that you start here:
Hey dw77, Did you find what you were looking for?
check amazon and search gor GREEN SCREEN. There are two highly regarded books on the market.