After a direct quotation or paraphrase you need to include an in-text citation.
If you include some of the required citation information in your introduction ("Smith and Jones conducted a study...") then you do NOT need to include that information in the parenthetical citation. In this case, the citation would just be (12). See the APA sample paper for more examples of parenthetical citation.
If you are citing an entire book or article (through summarization), then you do not need to include a page number in your citation.
A "References" page is included at the end of every APA paper. Citations follow this general format: