Academic writing is filled with rules. Knowing when and how to cite your sources shows that you can take part in academic conversation and avoid any charge of plagiarism.
No professor expects you to present your own ideas only; be clear about where your ideas, phrases, and special words come from. Any time you refer to another person's work--whether you quote directly from another's work, refer to another's work in your own words, or base your ideas on another's work--you must cite the sources you used. Citing your sources serves a number of very important purposes. For example, citing properly:
Plagiarism is the act of using another person's words or ideas without giving him/her proper credit. It is a serious academic offense.
The Academic Integrity page defines plagiarism in more detail and provides information about Muhlenberg's Academic Integrity Code. Check out Defining Plagiarism: A Guide for Students with helpful frequently asked questions.
Keep careful track of where you found your ideas, phrases, and key concepts so that you can correctly cite your sources. It may be helpful to use a citation management tool, like Zotero, to help you keep track of, organize, and cite your sources.