Define what plagiarism means. In order to combat plagiarism, you must know what it is. According to plagarism.org, plagiarism "involves both stealing someone else's work and lying about it afterward. "
Quote every phrase or line that you use from a source. If you place a phrase or line from a source without adding quotation marks around it, then you are trying to pass the phrase or line off as your own. When quoting, add a reference to who said it to give credit.
Write everything in your own voice. Do not copy anything from any other piece of source material. This includes speeches, TV shows and movies as well as any sort of written document.
Source external material that you use. If you do not source any of the facts that you state in your essays, then you are effectively copying it from them without giving them any credit. When sourcing external material, format it in the style used by your teacher. For instance, psychological papers are written in the APA format which uses its own style for sourcing external material.
Write some of your own fact-based opinions into your paper. According to plagiarism.org, if your paper borrows most or all of its ideas from just one other source, this is also a form of plagiarism as you have not added your own thoughts or ideas.
Run a plagiarism checker after you have written your paper. A plagiarism checker will detect your paper for any plagiarized material even if it was accidental. Open a plagiarism-based website like Article Checker, Plagiarism Checker or Plagiarism Detect. See the resources for links to these websites. Once the website of your choice is opened, copy and paste your paper into the text section and press "Scan." Once the scan is done, the program will tell you if and what was plagiarized.