Now remember if you only want to disable your account temporarily, then this is not how you do it. And after you've submit this request to Facebook team there's no turning back!.
1. First log-in to your account and from the right upper corner (as shown in below screenshot) choose: "Account" -> "Help Center".
2. Then from the "Help Center" window, into the search box at the middle, search for the below phrase.
How do I permanently delete my account?
3. Now you'll be taken into the Facebook Faq page and at the top will be the above phrase, simply click on it and scroll down until you find that which says "submit your request here", as shown below screenshot.
Now all you gotta do is think again if you really wanna do it... and if so, then simply click on that link and when asked confirm it by clicking on "Submit" button from the next window.
That's it. Now your Facebook account is officially deleted. Good for you :).