Use the basic format, giving the address in both the text and in the citation. For example, The American Academy of Pediatrics provides exceptional help with basic child-rearing information (http://www.aap.org/).
Modify the format if you know the author and date. In this case, the citation reads Author, A. (date on site). Document title. Retrieved from <list the URL>.
Cite the author, but no date using the full URL, the author and the title of the document. For example, American Academy of Pediatrics. Talking to Kids and Teens About Social Media and Sexting. Retrieved from http://www.aap.org/advocacy/releases/june09socialmedia.htm.
Cite the date, but not author using the full URL, the date and the title of the document. For example, Turnoff Week and Back to School (September 2009). Retrieved from http://www.aap.org/sections/media/TurnoffWeek09.htm.