This is the official APA home page. There is not a complete style guide given here, as they would prefer that you buy their printed manual. However, they do have a free tutorial and some basic support listed in their FAQ.
The OWL (Online Writing Lab) at Purdue is an extensive site for writing helps. In the right hand column of the home page there are links to APA, MLA, and ASA style guides and other useful writing information.