Format: 

Author, A. A. (Year). Title of webpage [Format]. Retrieved from URL

Examples: 

United States Environmental Protection Agency. (n.d.) Sustainability at the EPA. Retrieved from http://www.epa.gov/sustainability/

In-text: (United States Environmental Protection Agency, n.d.)

Notes: 

Entire websites do not need to be cited in your References list; instead, include the URL in parentheses within the body of your paper (see first example). If you are citing a specific page or document on a website, include an entry in your References list using the format in the second example. If the name of the website is not part of the author or webpage title, include that information in the retrieval statement (e.g. Retrieved from Site Name website: URL).

Source:
APA (ER) - p. 32