Your topic sentence is the most important sentence of your paragraph. It needs to be an opinion that can be argued. If it’s just a statement of fact, then there’s no need for any supporting sentences. For example, if you say:
Washington, D.C. is the capital of the United States.
then there’s no need to write anything further, no need for any supporting sentences.
But if you write:
There are three reasons why you should visit Washington, D.C.
then you are expressing an opinion in your topic sentence. And you can then develop this idea with your supporting sentences. You might talk about its history, how it’s the center of the federal government, about the museums and other landmarks there, etc.
Think about the city where you live. What are the unique features of your city? Try to create a topic sentence about your city.