My approach to this question in this time of the release of Steven Spielberg's movie Lincoln is to ask you to look at the book To Kill a Mockingbird with the same openness and ability to face the great racial rift in American society. Now to bring that into an essay is to look at how the book treats the trial, Atticus Finch, and the defendant. Think about whether you want to look at discrimination in the book through various characters with each important character having their own body paragraph. Perhaps you want to look at the effect discrimination has on the town itself and the people who live there. If I used this approach, I would pick out the most important examples of discrimination which affect the town and use each example as a body paragraph as I show its effects on the people. Make a list of what you see as discrimination, who is involved, what happens as a result, and then you can see which will be easier for you to write as an essay. List the body paragraph topic sentences with ideas you can use underneath. Then create the thesis statement which is in the introductory paragraph and covers the three main ideas of your body paragraphs. For example, 'The three most important people who illustrate the effects of discrimination in the novel To Kill a Mockingbird are Atticus Finch, Boo Radley, and Tom Robinson.' Then you have to prove this assertion using material from the novel. Good luck as this is an important essay about a very influential book, one from which we still have much to learn. Then watch the movie Lincoln and look at the similarities.
One way to decide upon a thesis for an essay is to take the topic and ask, What about it? With respect to discrimination, the student can consider social discrimination as encompassing what Scout calls the "caste system in Maycomb" and then ask, How does the social discrimination of Maycomb affect the maturation of Scout and her brother Jem? The answer to this question is, then, the thesis. Usually, the writer needs at least 2 or 3 points to discuss in the essay. For instance, Scout's exposure to the damaging effects of social discrimination makes her realize that all people deserve respect. In the last chapter, as she stands upon the Radley porch, Scout reflects,
Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them. Just standing on the Radley porch was enough.
This quote could even be used as a motivator or lead-in for the introduction; then the thesis formed around how Scout and Jem have learned to identify social bias and reject it through their association with such characters as Calpurnia, Miss Maudie, Mrs. Dubose, Mr. Raymond Dolphus, Mr. Cunningham, Tom Robinson, Bob Ewell, Boo Radley, and others.
See the link below for a critical essay on the "caste system of Maycomb." Good luck!