This is an open-ended question. If you need to write an essay on To Kill a Mockingbird, then you need a thesis statement. This is the most important part of any essay.
A thesis statement is debatable. It is an argument, not a fact. For example, you can say that Boo Radley is a mockingbird figure. Not all may agree with this statement. Another thesis can be "integrity and courage are the central themes of the book." Again, not all will agree. But you can argue for it.
What makes a thesis statement strong is that there is supporting evidence. The stronger the evidence is, the stronger your argument is. These arguments are in your body paragraphs. So, if you are writing on the topic of courage, you could mention Atticus's defense of Tom Robinson even though it was not a popular thing.
Finally, what makes an essay even better is when you are able to use quotes. Show how these quotes prove your points.