You need to cite your sources in your assignments because it provides evidence to support your arguments and because the sources' authors deserve credit for their ideas and hard work. If you do not cite your sources, you are stealing someone else's ideas and pretending they are your own, which is called plagiarism.
When you use information from one of your sources, whether you summarize, paraphrase, or quote it word for word, you need to cite the source in two places:
To create in-text and Works Cited citations in MLA style, use this guide.
To create in-text and References citations in APA style, use this guide.
If you do not know which style you need to use, consult your assignment instructions or ask your professor.