A topic sentence usually comes at the beginning of a paragraph; that is, it is usually the first sentence in a formal academic paragraph. (Sometimes this is not true, but as you practice writing with this online lesson site, please keep to this rule unless you are instructed otherwise.) Not only is a topic sentence the first sentence of a paragraph, but, more importantly, it is the most general sentence in a paragraph. What does "most general" mean? It means that there are not many details in the sentence, but that the sentence introduces an overall idea that you want to discuss later in the paragraph.
For example, suppose that you want to write a paragraph about the natural landmarks of your hometown. The first part of your paragraph might look like this: