You cut a mask or possibly try to key out the shape- hard to tell without a image to look at. Is the shot static or does the camera move?
If you need to cut a mask and you have a moving shot, you will need to track the shot and apply the information to the mask.
From the question I tend to think that you are new to AE. It would be beneficial to check out the tutorials here on the Creative Cow . Also, questions like this should be posted to this forum: