Joomla! is an open-source project that collects the ideas and experiences of hundreds of thousands of individuals, merges them, develops them further, and offers them online. You will always find the most up-to-date information online.
In the English-speaking world, your best place to start is the joomla.org website at http://www.joomla.org/.
It is divided into the following sections:
You will really find everything you need to successfully use Joomla!.
There is a well-maintained overview at the moment at: http://help.joomla.org/component/option,com_easyfaq/task,view/id,167/Itemid,268/.