The Purdue OWL is a very good resource for learning how to cite sources in the APA format. Its APA style page has links to two general areas: an overview and a workshop, and a style and formatting guide. Both of these sections provide detailed information.
The style and formatting guide gives a clear introduction and explanation of the various kinds of APA citations, and teaches how to use in-text citations, footnotes, and endnotes, and how to organize them in a reference list. The guide also points out common mistakes, and it provides many tips and tricks to make it easy to format citations properly.