When brainstorming, you list every idea that comes to mind without evaluating its merits. Your goal is to develop a sizable list of topics quickly; later, you will consider which one would be best. Do not censor any idea at this point in time: just let your thoughts flow.
To brainstorm, consider your interests and experiences, issues you care about, organizations you belong to, people you admire, events you find significant, places you have been, and lessons about life you have found important. Think about favorites in each of these categories. For example, what is your favorite use of spare time? The most interesting course you have ever taken? The best organization you’ve ever belonged to? By focusing on questions like these, you will soon build up a strong list of potential topics.