A topic sentence is a generalization of your paragraph.  Think of it as the introduction to your paragraph, like the introduction to your essay introduces your reader to your essay.

This point should be formulated into a topic sentence, a general sentence that provides a very broad, sneak peek into the specifics of your paragraph. (enotes how to write a perfect paragraph)

This sentence should be one of the points of your essay.  It is best to arrange your essay around a central idea (the thesis) supported by three supporting points or arguments.  These are your supplemental or body paragraphs.  You can have more than five paragraphs in your essay, but you should not really have more than three basic points.  If you have more, chances are they are subpoints and can go under one of the other points.

Once you have chosen your points, the first sentence of each paragraph is very important.  You want this sentence to give a specific hint of what will be in the paragraph.  Let’s say you are writing about why your favorite food is ice cream, and your three reasons are: 1) It has calcium, 2) It tastes good because it is sweet, and 3) It is cold and refreshing on a hot day.  Your topic sentence might be as simple as:

First of all, ice cream is my favorite food because it is healthy due to the high calcium content.

This is an overly simple topic, but you can see how the premise works.  In one sentence, I have told you what the topic of the paragraph is, reinforced the thesis of the essay, and given a brief explanation that I can easily go on to support.

Developing good topic sentences gives you a road map for writing the paragraph and makes writing much easier.