Cite quotes, other people's ideas, and facts that are not common knowledge.
As a rule of thumb, there are three things that need citations in academic writing:
Summaries of other people’s ideas
Specific details and facts (like statistics)
There are two things that don’t need citations:
Your own ideas (including the results of your own research)
Facts that are common knowledge for your intended readers
For every source you cite, include a reference in your reference list.