Basics of APA Style tutorial produced by the American Psychological Association.
The 6th edition of the Publication Manual of the American Psychological Association (released in June 2009).
In particular, see these slides in the Basics of APA Style tutorial:
Slides 1 and 2 Introduction to APA Style
Slide 3 - 6 Formatting a paper
Slide 7 and 8 Headings
Slides 13-16 Citing references in text
Slides 17-25 Reference List
Slide 26 Additional Resources