Citations and attribution are important parts of research. Citations boost your credibility by showing your audience where you got your information from, they allow us to give credit to others for their work, and they are necessary to avoid plagiarism and intellectual theft.
You need to cite anything you didn't create, including:
You may not need to cite "common knowledge", which is facts or concepts that are widely known, like the Earth is round - however, what is considered common knowledge varies depending on culture and context, so be careful with this.
When in doubt, just cite it!
You can always use the library search or a database to automatically generate a citation, but make sure you double check that it's accurate! Automatically generated citations tend to have errors, like missing author names, incorrect page ranges, or unnecessary information.
Use the resources below to double check that your citations are accurate and contain all the information you need.
Publication Manual of the American Psychological Association by 
MLA Handbook for Writers of Research Papers by 
The Chicago Manual of Style, 17th Edition by 