Plagiarism -- It is extremely important to make sure that your writing does not use someone else's work without properly acknowledging it. Even "paraphrasing" (putting someone else's writing into your own words) must be done in a very careful way.
These tutorials will help you learn the basics of avoiding plagiarism. You will always need to know this, not only during your academic career, but throughout your professional career, too.
Also, here is information from the Office of Scientific Integrity at DHHS (Department of Health and Human Services).