The first step in avoiding plagiarism is understanding what it is. Students have been known to commit "accidental" plagiarism, because they did not fully understand the rules regarding acceptable and unacceptable academic writing practices.
Generally speaking, plagiarism is easily avoided by using proper writing techniques such as paraphrasing, quoting, and referencing and citation to indicate whichever information and ideas used in a writing project are taken from outside sources.
For more information on plagiarism and how to avoid it, see these recommended resources: