First and foremost, to avoid plagiarism, it is vital that you familiarize yourself with and utilize proper citation techniques when summarizing, paraphrasing, or quoting someone's ideas.
It is also beneficial to keep in mind safe practices that help you avoid plagiarizing when researching and writing papers.
For information regarding safe practices see Purdue's Online Writing Lab, where the website demonstrates how to efficiently conduct research as well as how to properly incorporate quotations, summaries, and paraphrases in your writing.
The KU Writing Center also addresses the proper way to paraphrase (http://writing.ku.edu/paraphrase-and-summary) and incorporate references into your writing (http://writing.ku.edu/incorporating-references).