Gather reference information. As you're researching your essay, make sure you collect bibliographic data on each source you intend to use. You'll need to know the author, date, title, publisher and place of publication. For journals, note down the volume and issue number. Record the page number of any fact, statement or quote you intend to reference.
Check your school, university or department's referencing style. There are a wide variety of different referencing formats, including those published by the American Psychological Association (APA) and the Modern Language Association (MLA). These styles are common in the humanities, but many scientific fields have their own reference formats.
Insert citations into your essay as you write. There are two main ways of inserting citations. You may use in-text citations. In an in-text citation, a statement is followed by the author and date of the source from which it comes. For instance, you might write "traffic accidents declined by 5 percent between 1998 and 2000 (Jones 2003)." Alternatively, you might reference with footnotes or endnotes. In this case, instead of placing the reference in the text, you would insert a small superscript number pointing the reader to a reference at the bottom of the page or the end of the essay. Which style you use depends on your referencing format.
Add a bibliography to your essay. This consists of a list of the works you have cited, organized alphabetically by the last name of the author. The exact format of your bibliography will depend on the reference style you're using.