Plagiarism is the practice of copying large chunks of text or the using pictures from books, the World Wide Web, or any other source (e.g. CD-ROM) into your essay without saying that you have used that source. Any work that is not your own: e.g. arguments, quotations, photographs, drawings etc., MUST be acknowledged at all times. Other people's work is protected by copyright law. It is a very serious offence to simply copy their work without acknowledging that it is someone else's work and not your own.
It is also not appropriate to submit the same piece of work twice for different courses.
If you have difficulty putting things into your own words then ask your teacher, your parents, a friend for some help. There are also a large number of online resources and books which would assist. Ask your teacher for recommendations.