Unfortunately it’s not possible to fully delete your account. After you deactivated it, it will be (nearly completely) deleted after 24 hours. As the site tells: certain data may be retained on the site, such as comments you made. If you have problems with that, the best you could do is anonymise your account first and then delete it.
Be logged in. As stated, if you need to delete things such as comments, try to do so or at least anonymise your account (fill in a nonsensical name, etc.). Then, at the top of the screen: click My profile. Scroll down. There you’ll see the deactivation link. From there it’s easy.