Since prejudice is the major theme of Harper Lee's "To Kill a Mockingbird," you may wish to build your essay around the character of Mayella Ewell since she is the catalyst for the action of the second part of the novel. Also, Mayella represents the beliefs and discriminatory views that contribute to the town's persecution of Tom Robinson.
For one thing, although the Ewells live beside the garbage dump, in the most sordid of homes in which Mayella is a virtual slave herself without friends and subjected to sexual abuse by her own father, the Ewells feel that they must maintain a superiority over a black man. So, they falsely accuse Tom of inappropriate behavior. Mayella must perpetuate the myth of southern womanhood.
Another contribution that Mayella makes to the persecution of Tom is in her putting Tom into a dangerous situation for a black man. When she makes sexual advances to him, Tom testifies that he did not push her away because he did not want to hurt her. After he admits feeling sorry for a white woman, the white males on the jury interpret Tom's behavior as exerting a sense of superiority toward the poor young woman. This act is unthinkable, and in the men's eyes, it muct be paid for by Tom with his life, if necessary. Again, the myth of southern womanhood must be preserved as well as the superiority of whites over blacks.
Clearly, the theme of racial attitudes is demonstrated with the character of Miss Mayella Ewell whose testimony underscores the "usual disease of Maycomb," as Atticus has said.
Racism is a central theme in "To Kill A Mockingbird". Unfortunately for you, racism is a very BROAD topic. The first thing you want to do is narrow it down. Try, "Racism in Harper Lee's To Kill a Mockingbird" shows the true nature of many charcters. From that point, you explain where racism occurs in the text, and then you discuss what racism and racist thoughts and behavior tell the reader about who the characters truly are. Basically, most of the characters Scout knows are not racist, which is why her encounters with racist people are so shocking. It is her innocence that is so informative.