Sometimes choosing a topic is easy. Something strikes your interest, or your professor guides your decision in some manner.
Sometimes, especially if it's a discipline with which you are unfamiliar, or an area that doesn't readily grab your interest, finding a topic can be more challenging.
In the latter case I like to start broad, and then ask myself a series of questions until I find a more narrow focus that interests me.
NOTE: It is critical to find something that interests you. The more motivated you are to sincerely learn about a topic, the easier it will be to do the work of researching, learning, and reading about that topic.
But, if I'm not sure what interests me, I start with a broad element that strikes my curiosity at least a little, and then I ask questions about it.
"Research is iterative and depends upon asking increasingly complex or new questions whose answers in turn develop additional questions or lines on inquiry in any field."*
At this point I might even browse the internet, or read the entry at Wikipedia. And, the questions I ask myself are typically open-ended. I want to ask How? or Why? or Should? If I only ask closed-ended questions (Who? When? Where?) I find the answers quickly and don't have a topic to write about.
Once I start getting traction on something that might be interesting, I'll then typically look to see if I can find a book on that topic, or do some cursory searches in Summon or Google Scholar. At this point, also, it might be a good time to check with your professor to see if you're heading down the right path.
*"Framework for Information Literacy for Higher Education", American Library Association, February 9, 2015. http://www.ala.org/acrl/standards/ilframework (Accessed June 19, 2020.)
Focus on a specific aspect of your topic. Follow these helpful steps:
Steps in the Research Process: