To properly cite "Web documents," you first must be familiar with the basics of APA Style, available from this page.
To cite online materials, first differentiate between
If you have used a copy of an article imbedded within an "article database," such as LexisNexis Academic or Academic Search Complete, use Example #20.
On the other hand, if you have retrieved an article directly to its publisher's Web site, such as the New York Times' own site, or some organization's site, use Example #19.
This category is "everything else" -- documents, reports, essays, and other items found in sites created by educational, commercial, or non-profit organizations. In such situations, use Example #21.
Please note that there are specialized recommendations for citing Government materials.
For additional information, check the Electronic References area of the APA Web site.