Joomla Tutorials helps you to learn all the aspects of using Joomla. The site also helps you to learn about Joomla when you are first introduced to the site.
I highly recommend this website to every Joomla Beginner because this site doesn't just provide articles, but it also provides a visual video of someone showing you how to manage your Joomla site. It provides training, templates, etc.
Joomla.org provides training, templates, downloads, and you can even visit the community portal when you hover over the Community & Support on top.
This article gives you the ten best ways to keep your Joomla site secure.
If you want to go more in-depth with the Joomla security, I provided an article that discusses 12 Best Ways to secure your Joomla site.