Hey there!
Topic sentences are often used to begin paragraphs, as they tell what the paragraph is about. If you are writing an essay about how to grow plants, then your topic sentences may be about things such as digging holes to the proper depth depending on the plant, sunlight for each plant, water, weather, etc. If you are looking for a thesis statement, which would be the overall topic of your essay, then you would want to talk about growing plants in a more general sense. For example, you could say something about how anyone can grow plants, and that growing plants only takes a few simple steps (you would list out the steps in your thesis statement). Then, taking those steps, you could discuss each step in a different body paragraph.
Hope this helps!! :)