Joomla Documentation

Joomla Administrators Manual - https://docs.joomla.org/Portal:Administrators

Beginers guide to Joomla - https://docs.joomla.org/Portal:Beginners

Joomla Security

Security Checklist - https://docs.joomla.org/Security_Checklist

Security and Performance - https://docs.joomla.org/Security_and_Performance_FAQs

Security Extensions

jHackGuard - https://www.siteground.com/joomla-hosting/joomla-extensions/ver1.5/jhack.htm - VERY useful. Pay attention to the installation guide, especially adding/whitelisting user groups or you'll lock your website.

Brute Force Stop - http://extensions.joomla.org/extension/brute-force-stop

Spam

Google reCAPTCHA - https://www.google.com/recaptcha/intro/index.html - Install and enable for use with default contact form and registrations.