You may use books and Web sites to help you get information for your paper, but don't copy words from your sources into your paper unless you are clearly marking them as quotations and adding a footnote.
Even if you copy and paste and then change some of the words so that they don't look exactly like the original source, you have still taken another author's work away from him without giving him any credit for it. You have created something called a derivative work, which is a form of copying according to United States copyright law. Take a look at this article on plagiarism and copyright if you don't believe me.
Except for quotations, all of the words you write for your paper should be your own words. Anything else is plagiarism.
Teachers can check for plagiarism using the tool at PlagiarismChecker.com. All they have to do is type in a couple of short phrases from your paper and hit the Search button. You should try this yourself to see how easy it is.
If your teacher finds part of your paper on PlagiarismChecker.com but you still want to say you didn't copy it, then try this: Calculate the probability that you would write exactly the same words as the author of a Web site.
Assume that the English language has a vocabulary of 1000 words, which is really much smaller than it should be. (Hey - I'm trying to give you the benefit of the doubt!) The probability that you didn't copy from the Internet is
The math will show you the probability that you would write exactly the same words that someone else did without copying. The chance is very small. For a ten-word sentence, the probability that you didn't plagiarize would be about 0.000000000001%.
Maybe you wrote your own paper honestly, but some of the words show up on the Internet anyway. It really was an accident. How can you convince your teacher that you didn't copy?
You'll have to prepare in advance if you want to be ready. Save copies of all your research notes and rough drafts. Those notes can serve as evidence that you wrote your own paper. Above all, if you copy any words directly from a book or Web site into your notes, highlight those words or draw a box around them. This will keep you from accidentally putting someone else's words into your paper.
Once you show your teacher all of your notes, she may be impressed at how careful you were. She could be less likely to accuse you of plagiarism.