This video covers the basics of how many people accidentally (or purposefully) plagiarize.
1.) You must place any quotations in quotation marks.
2.) You must cite your sources using an appropriate citation style (MLA, APA, Turabian, Chicago, etc.)
3.) Don't quote Wikipedia in an academic paper. It's not plagiarism - Wikipedia's just not a reputable, reliable source of information.
4.) A paper that is cobbled together from a series of sources without any narrative or argument that ties them together is going to fail, and could be considered plagiarism.