For practice help in learning English grammar and sentence structure, I recommend the Guide to Grammar and Writing, link to it here: http://grammar.ccc.commnet.edu/grammar/
To plagiarize is to use someone else's idea without giving that person credit for coming up with the idea. There is a very good tutorial from the University of Hong Kong that can help you learn about plagiarism and how to avoid making a plagiarism mistake. I recommend you work thorough the tutorial, which you can find here: http://www4.caes.hku.hk/plagiarism/content_brief.asp