Identify your topic. This is the broad subject you will discuss or have been discussing as part of a larger essay. For example, your topic may be junk food.
Identify the controlling idea. The controlling idea is what you want to say about the topic. For example, if your topic is junk food, the controlling idea may be types of junk food, the effects of junk food on health or the amount of junk food children consume.
Brainstorm or research supporting details to include in the paragraph that explain or develop the controlling idea. These details won't necessarily be included in the topic sentence, but they will help you write it. Since the topic sentence is a "road map" for the paragraph, you have to know generally what the paragraph will contain in order to write a topic sentence that accurately reflects what follows. If your paragraph is about types of junk food, your details may include candy, hamburgers, French fries and chips.
Take a step back and look at the big picture. Decide whether the controlling idea and supporting details truly align with your thesis statement, if the paragraph is part of a larger essay. If the paragraph will stand alone, ask yourself if the information that you've brainstormed so far accurately represents what you originally wanted to communicate.
Look for similarities among the details you listed in Step 3. The goal is to find phrases to include in the topic sentence that summarize the details succinctly. For example, if your list of details includes soda and Kool-Aid, you can describe both details by simply saying "sugary drinks."
Write the topic sentence. The sentence should clearly indicate your topic and controlling idea and set realistic expectations for what the reader will encounter in the paragraph using the phrases you brainstormed. For the junk food example, "There are many types of junk food" is an accurate topic sentence, but it's much stronger to write, "Junk food consists of any food without significant nutritional value, such as candy, processed food and sugary drinks." The paragraph can then provide specific examples of candies, processed foods and sugar-filled beverages.