Hi
Check out this for a start.... http://nch.invisionzone.com/topic/25627-using-mask/
However, this is not quite the same as your problem as it shows the set up for a fixed image (s). Masking a moving image or object that has a complicated and moving outline can only be done successfully if it was filmed against a green screen. Also, if you have only one clip then although you can mask out yourself from it (provided you don't move) you would need a separate clip of the moving background without yourself on which to place your masked image.
Nat