WordPress Codex is the Holy Bible for WordPress documentation.
There are tons of pages with documentation and it is categorized really simple so you can quickly refer to any section regarding what do you need.
You can find “WordPress for Beginners”, “Designing Your WordPress Site“, “Lessons for Theme Development” and “Website Development“.
The information in this sections is a great resource to become a WordPress expert starting from zero, is an online manual that WordPress itself, thru their developers maintain updated with all the information.
You can learn how to install, and update WordPress, and later going thru the first steps with WordPress.
There are lessons and tutorials that you can find, and a Glossary in case you don’t understand a specify term, so you can jump right into the glossary and see the definition.
Also you can find any errors you may have in your installation or while you are setting up your site, or later while you are actually using it, in the Codex you while find a list of errors so you can fix any problem you may encounter.
Also if you are interested in Themes or Plugins development, you will find a complete set of information and concept like Child Themes, template hierarchy or template tags. And for plugins you will find the API information and the coding standards.
Is there big amount of information, that is why it may seem unorganized or not easy to find answers, but the search and the forum is helpful, although there are users that will refer to the Codex answer when you have questions, make sure to use the search before making any questions, it may exist a simple solution and you will not need to wait until someone responds.