• When a reference has three or more authors, cite the family name of the first author followed by et al. and the year, e.g.

(Ng et al., 2007)

• When two sources would both abbreviate to the same “et al.” form, include as many surnames as needed to differentiate between them.

• If two or more sources have an identical author/authors and publication year, add letters to the years, e.g.
  (Smith, 1991a).

(Coates, 2005; McMinn, 2003; Ng & Leung, 2007).

• When citing multiple sources in one in-text citation, place citations in alphabetical order. Separate citations with a semi-colon, e.g. (Coates, 2005; McMinn, 2003; Ng & Leung, 2007).
• Include page numbers for direct quotes, eg. Forester (1999) states that “If curiosity kills cats, presumption kills negotiators” (p. 147).

Read more at the source (University of Melbourne)

In scholarly writing, it is essential to acknowledge how others contributed to your work. By following the principles of proper citation, writers ensure that readers understand their contribution in the context of the existing literature—how they are building on, critically examining, or otherwise engaging the work that has come before.

APA Style provides guidelines to help writers determine the appropriate level of citation and how to avoid plagiarism and self-plagiarism.

APA guidelines also provide specific guidance for in-text citation, including formats for interviews, classroom and intranet sources, and personal communications; in-text citations in general; and paraphrases and direct quotations.

Find specific guidelines for in-text citation in the APA Style Guide.

We suggest:

Paraphrasing
Quotations
Quotations from research participants
Personal communications 
Classroom or intranet sources

It may be useful to examine sample papers, eg. sample student paper with citation annotations.