This will mask out the extraneous parts of your shot.

Next draw a rough mask around your subject while leaving enough room to contain any of their movement.Select your footage then select the pen tool.This is a mask that is big enough to contain our subject but small enough to remove parts of the shot you want to get rid of. To get rid of this, create a garbage matte around the subject. Your shot will often have objects on the edge of the frame you want to remove such as lights or stands. Often times a green screen shot will not make use of the entire frame. You should now see Keylight listed along with it’s settings.

Now select your footage and open the Effect Controls panel. Select Keylight 1.2 and drag it to your footage. Then go to the Effects & Presets panel and search for Keylight.