Go to My Account > Profile > Account Info.
Click Deactivate / Delete Account.
The system will ask you if you want to deactivate or delete your account.
Deactivating your account will remove your name from the directory but keep a record for the admin in case you want to reactivate your account. When you deactivate your account, you can choose if you want to have all your posts be deleted or not.
Deleting the account is a permanent action, and your account cannot be recovered. All your posts will be deleted when the account is deleted.