Note down as much bibliographic information about your source as you can. If it's a personal conversation, log the date and time. If it's a speech, do the same and try to get a title.
Check the citation style used by your university, school or department. There are a variety of different citation styles, each with its own rules.
Insert a citation into the text of your essay. This can take the form of an in-text citation, a footnote or an endnote, depending on the rules of the citation style you're using. Many citation styles use the term "pers. comm." for "personal communication" to signify information gained from talking to a source.
Add a reference to your "Works Cited" section. In the case of personal communication, this may not be necessary, but for interviews or lectures it sometimes will. Again, the exact specifications will depend on the citation style you use.