To narrow a topic that is too broad, try to pick one sub-topic of the topic to focus on. If you are having trouble thinking of a sub-topic, try asking yourself questions about your topic. The answers to those questions can reveal sub-topics that might offer a possible route to narrow your topic.
Answer: well, if your topic is about sports, then narrow it down to just one sport, or even a specific person playing that sport. Think of things inside your topic, smaller things that would be easier to write about