Once you understand that plagiarism is using another's words or ideas and claiming them as your own, you should have an idea how to avoid plagiarizing. When you use someone else's words or ideas, you must acknowledge him or her.
When you want to use a phrase from a book, website, movie (or anything with a fixed form) you must use quotation marks to indicate that it is a quote. You must also acknowledge the source with a citation.
When you paraphrase, you must rewrite in your own words. Simply changing or moving words around is not paraphrasing. The grammatical structure of the passage should be significantly different!
Still not sure how to avoid plagiarism? Look through some of the examples on the following guides: