Any time you reference, quote from, or use an idea in another source when composing an essay, you must cite that source--even if you were the author. Failing to do so is called self-plagiarism, and self-plagiarism's consequences are similar to regular plagiarism. You can suffer a point deduction, a failed assignment, removal from a class or expulsion for plagiarism, depending on your institution's rules. While citing your previous essay can be difficult--especially if it is unpublished--you need to learn how to do so in order to avoid the costly punishments of plagiarism.
Find the publication in which your essay was initially published. Note what type of publication it is. The most popular citation styles, including the Modern Language Association (MLA) and American Psychological Association (APA), have different formatting guides for different kinds of publications, such as books, magazines and electronic sources.
Create an in-text citation for your previously-written essay. In parenthesis next to the information you are quoting, paraphrasing or summarizing from your previous work, insert the author's (your) last name. If you wrote the essay with another author, include his last name as well. In APA style, follow the authors' names with a comma and the date, as well as another comma and "p." plus the page number if the information cited is a direct quote. In MLA style, do not use a comma and write the page number.
Cite your previously written essay on your works' "cited" page for MLA; or "reference" page, for APA. Use your style handbook to find the citation format for the type of source you are using. For example, if you published your essay on your website, in MLA, you would write your last name, a comma, your first name and a period. Then you would put the title of your essay in quotation marks, followed by a period, the site name (you.com) in italics, and another period. Next, you would write the name of the publisher, a comma, the date of publication, the medium of publication, a period and the date you most recently accessed it.
If your essay has not been published and is a thesis or dissertation, follow Step One above to draft in-text citations. Next, follow your citation style's standards for citing unpublished theses and dissertations. For example, in APA, cite an unpublished dissertation by listing the author's (your) last name, a comma and your first initial, followed by a period. Write the year of completion in parenthesis, a period, the title of the dissertation, a period, the words "doctoral dissertation" in parenthesis, another period, the words "retrieved from" and the name of the database that stores it and the dissertation number in parenthesis.
Ask your instructor how she prefers you to cite an essay that has not been published and is not a thesis or dissertation. Unpublished essays are not common resources, so most citation styles do not deal with citing them.
If you are citing multiple works that you have authored, use your citation style's method to clearly note the works' differences. For instance, in APA, the date differentiates works by the same author in in-text and reference page citations. In MLA, cite the title of the work along with your last name in the in-text citation to show readers which citation on the works-cited page you are referring to.