Click "deactivate" to the far right at the bottom.
It will ask, "Are you sure you want to deactivate your account? Deactivating your account will disable your profile and remove your name and picture from anything you've shared on Facebook."
Then it may pick a friend of yours and say "SoandSo will miss you." Facebook will even display his or her photo, in an attempt to make you feel warm and fuzzy about the service you are trying to leave. It may even tell you how many friends you stand to lose!
You must answer two more questions before you can click the button to deactivate.