What is plagiarism? Broadly speaking, plagiarism is presenting another's work as your own. Every academic institution requires students to properly credit work that is not their own. You do this by citing sources that you've used (the links to the right discuss how to properly do that in your papers).
Zotero is a tool for collecting, managing, and citing sources. It is a more robust, academic citation manager than most of the free web-based citation tools, as it was built by George Mason University's Center for History and New Media. But, it is still free, cross-platform, and open source. Check this guide out for an in-depth look at using Zotero.