I can remember writing in school and having teachers tell me to narrow my topic. Hopefully, this prompt will help you do that with your writing.
So…choose a topic…say, "education" for example. And, write it at the top of your page. Then, write down as many narrower topics that you can think of that have to do with your broad topic. For example, for the broad topic of "education" you may write things like: special education, science, favorite teacher, how to study for a test, etc.