How to take a screw out of my laptop casing that has no head?

Since the screw was stuck in the socket to begin with, these might not be too successful, but you can try using a rubber band over the screwdriver tip to grip the stripped head of the screw:

http://lifehacker.com/5462520/remove-a-stripped-screw-with-a-rubber-band

There are some other tips at Lifehacker, but a lot of these seem to be geared toward larger screws:

http://lifehacker.com/5901738/how-to-remove-screws-with-stripped-heads

There are kits you can get to drill out stripped screws at hardware stores.

If you have a cutting wheel for a small rotary tool, cut a flat blade notch into the outer part of the screw then you can use a flat bladed screwdriver. If you're careful, you'll only ruin the screw and not the plastic of the case. The screw is stuffed anyway so there's no harm is making it worse.