Support for the Joomla! CMS can be found on several places. The best place to start would be the official Help Site. Here you can help yourself to the information that is regularly published and updated as Joomla! develops. There is much more to come too!
Of course you should not forget the Help System of the CMS itself. On the topmenu in the Back-end Control panel you find the Help button which will provide you with lots of explanation on features.
Another great place would of course be the Forum . On the Joomla! Forum you can find help and support from Community members as well as from Joomla! Core members and Working Group members. The forum contains a lot of information, FAQ's, just about anything you are looking for in terms of support.
Two other resources for Support are the Joomla! Developer Network (JDN) and the Joomla! Extensions Directory (JED). The Developer Network for example provides lots of technical information for the experienced Developer as well as those new to Joomla! and development work in general. The JED whilst not a support site in the strictest sense has many of the Extensions that you will need as you develop your own Web site.
The Joomla! Core members are regularly posting their blog reports about several topics such as programming techniques used in Joomla!, Security, Events and more. You can even subscribe to get notified by e-mail when a new blog post is made!
Joomla! Documentation can of course be found on the Help Site. You can find information such as an Installation Guide, User and Administrator manuals, Frequently Asked Questions and a lot more. The User Documentation Team are the guardians of the documentation relating to the practical use of Joomla!. The Developer Documentation Team are the stalwarts behind detailing how Joomla! works from the ground up and how you can develop your own Extensions and Templates and much more.
Other than the documentation provided by the Joomla! Documentation Work Groups there are also books written about Joomla! You can find a listing of these books in the Joomla! Bookshelf.