Read through your assignment instructions carefully. Instructors may have assigned a topic or a general subject.
List any ideas which come to mind.
Try to take your ideas and narrow them down to a more specific topic. For example, instead of writing about "politics" which is very broad or even "Congress" which is still a very general, narrow your topic down to something such as "controversy caused by the Patriot Act" or "how the United States has become more politically divided in the last 50 years".
Do a quick search or two to make sure that there are resources available for that topic. You do not want to choose a topic and later be frustrated because it is difficult to research.