Use your own words and ideas when you write. As a rule of thumb, use other sources of information to support your own ideas rather than as a replacement for your own writing and thought process.
Cite your sources. Use in-text and parenthetical citations to give credit for words and facts that are not your own: In other words, include such information by merging it smoothly and sensibly into the body of a sentence you have written or by enclosing it separately but still within the sentence by using brackets or round parentheses. Make use of footnotes, references and/or a bibliography, depending on the type of information you have used and whether or not you are writing a formal document, paper or essay. Look online to find ways of presenting such information or ask a professional: Librarians, teachers or professors may be able to guide you.
Paraphrase from the original source rather than using exact phrases, sentences or paragraphs. "Paraphrasing" does not mean copying from the source and adding just a few minor word changes. It means using your own words to express ideas that are not your own. Use more than one source, if possible, to support your writing -- especially on larger pieces, or to express bigger or related concepts, you will add credibility to your writing by showing your effort at research. Evaluate and understand source information. Use sources to support, compare or back up your own ideas. Use facts and ideas you have found and work them, in context, in your own words, into what you want to express in your writing.
Use quotation marks when quoting directly from a source using its exact words. Do not quote more information than is necessary. Reference the author's name in the quoted sentence whenever appropriate; you may possibly want to include the year in which the author published next to the name, depending on the kind of information you included or the kind of document you are writing. Use ellipsis points [three dots] to shorten long passages, thereby leaving out unnecessary information.
Cite facts that are not common knowledge. For example, it is common knowledge that the Earth travels around the Sun. There is no need to tell anyone how you know that. The exact distance of the Earth from the Sun at its farthest point is not common knowledge. You may find that information in innumerable sources. Pick one source and tell the reader where you found such specific information.
Proofread your writing. Check not only for spelling and grammatical errors, but for missing quotation marks (there should be quotation marks at the start and end of each quoted word, phrase, sentence or paragraph you have used from a source), missing citations and bad paraphrasing.
Use a plagiarism checker. Plagiarism tools and services, such as Copyscape or Grammerly, can check your writing for exactly copied text. You can find a number of these tools online by typing "plagiarism checker" into a search engine's search box.