Something that is controversial is an object of debate. There are viewpoints that are in conflict. The word "controversial" is formed from the root words contra and versus, which gives you a sense of the conflicting perspectives.
Background research: learning about the topic and the main elements
Tip: try a database in the page "Find Information For / Against"
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Consider what statements are Facts and which are Opinions
Example: the number of students enrolled at a university is a fact. Whether the number means the university is popular is an opinion.
Does your assignment ask you to draw a conclusion, present your own opinion, or just gather the points of the debate?
The purpose of your assignment will influence how you put together your project (written or otherwise)
Tip: for writing help, try the link in the left navigation menu titled "Research & writing help"
Sometimes topics are assigned by the instructor. If not, and you need to choose one yourself, consider picking something that interests you but about which you're not an expert. This way you'll have motivation to investigate the topic and enough unknowns to help keep the search interesting.