Username

Password

E-mail address

Type "I am one hundred percent sure that I want to deactivate my account." without the quotes.

Before continuing realise that deactivating your account will remove you from every rank you're in and active Tenshi will not pause.
If your e-mail address is valid you can reactivate your account using the "Resend Activation E-mail" form on the authentication page.
Forum posts and other data you've published on the site won't be removed upon deactivation, if you want these gone you can go through them yourself.

TL;DR: Deactivation removes all ranks from your account and won't be readded (except automated Tenshi) even if it was a joke.