You cite your sources:
Plagiarism is presenting another person’s words and/or ideas as your own words/ideas – either deliberately OR unintentionally.
To avoid plagiarism, give credit in your paper to the person whose words and/or ideas you have made use of. In other words, cite your sources. You must cite any source that contributed significantly to the ideas in your own paper, even if you don't quote directly from that source.
See Plagiarism Prevention from OnlineColleges.net for more information.