Are you trying to stabilize and crop the character?
Or follow the edge of the character and cut him out in detail?
From the sounds of it you're trying to do something relatively difficult for someone new to after effects. Those animation sequences usually have a high rate of change so the odds are you'll have to cut the character out frame by frame using masks.
Please provide at least a single frame of the animation for further feedback.
Mike Sevigny
TorusFX Inc.
http://www.torusfx.com
http://www.linkedin.com/pub/mike-sevigny/49/875/495