be sure to block the Facebook from your browser too if you really want to stop using it. The problem with deactivation is that many people start to use it out of habbit and go back.
Just go to the account settings and click on 'security' and at the bottom it says Deactivate my account. Click on it and it will ask you why, ask for your password and to type in one of those impossible to read CAPTCHA security codes that will take like 20 tries, then it will be deactivated. They will send you an email confirmation, and then if you ever want to reactivate it, simply log in with your user and password and it's reactivated.
Once you click the settings at the right corner of your profile, you can find the account settings-> Deactivate the account at the bottom of the page, that's it......
Just don't forget to download a backup of your information via Account Settings. Unless you don't want to remember any message you've ever written or received...