I think it would be relatively easy to argue the following thesis: Harper Lee shows that those who are racist are ignorant, uneducated, and unkind through her depiction of Mrs. Dubose, Mr. Ewell, and Mrs. Merriweather.

It is not very difficult to find examples of bad behaviors exhibited by these characters which show that we, the readers, are not supposed to relate to them or like them. We might sympathize, perhaps, with Mrs. Dubose, but it is clear from her behavior toward Jem that we are supposed to learn compassion from Atticus's treatment of her rather than to emulate her ideas. Mr. Ewell is portrayed in an even worse way, as a child abuser and liar and would-be murderer. By discouraging readers from liking these characters, Lee encourages us to condemn their racist beliefs.

You could also argue the flip side of this idea, which is that Harper Lee encourages readers to like characters like Atticus Finch and Ms. Maudie in order to encourage our disavowal of racist ideas.

A thesis is the main statement of an essay that controls the direction the writer will take while writing. It informs the reader about the specific theme and details to be discussed. One way to write a good thesis statement is to present the chosen theme by using direct language. Then, follow it up by adding three supporting details listed thereafter. For example, if the theme is racism as presented in To Kill a Mockingbird, think of three events that demonstrate racism or three characters who either dish it out or have to take it. Then, list those events or characters right after the thesis statement. The examples listed with the thesis statement will then be discussed in further detail in the body of the essay. Below are a few examples:

1. The people of Maycomb who exhibit racist behaviors are Mrs. Dubose, Mrs. Merriweather, and Bob Ewell.

With this thesis statement, the writer establishes that racism does exist in Maycomb, which is the theme, and then lists three examples of characters who demonstrate it in the novel. The essay would then explain in detail what those characters say or do.

2.  People in the town of Maycomb, Alabama show their racist attitudes by not hiring Helen Robinson when her husband is in jail, attempting to lynch Tom Robinson before the trial, and convicting him for a crime they know he didn't commit.

An essay written from the example above accuses groups of people for demonstrating their racist attitudes because a black man is charged with allegedly raping a white woman. However, these groups create specific events that can be referred to in the book as evidence for the essay. 

It's easy to write a thesis statement if you understand what it is.

Your thesis is your main idea of your essay, and your thesis statement must state your main idea. It's a good idea to plan out the points you will use to prove your main idea, and you should include the points in your thesis statement.

I know what your theme is, racism, but I don't know your thesis. Your thesis should be a statement about your theme. So you might say "Racism is the main theme explored in To Kill a Mockingbird" or you might say "Harper Lee is against racism in To Kill a Mockingbird." There are several other ways you could state a thesis about this theme, too.

Let's go with the first thesis, "Racism is the main theme explored in To Kill a Mockingbird."

Now you need to plan out your essay and decide on some points you can make to prove your thesis. A good way to do this is through brainstorming. You should come up with lots of points, and then check through the book and figure out which points would be best to support your thesis. You should be able to find evidence to support each point. You can find more on themes in To Kill a Mockingbird in eNotes' study guide, here. You should choose at least three points to use in your essay.

Here are my suggestions for points to support the thesis "Racism is the main theme explored":

• The majority of white people in Maycomb are racist.
• Maycomb is separated into white and black neighbourhoods.
• Tom does not get a 'fair' trial because of racism.
• The word "nigger" is used a lot in the text, and Scout and her father are called "nigger lovers."
• People in Maycomb have "...the evil assumption—that all Negroes lie, that all Negroes are basically immoral beings, that all Negro men are not to be trusted around our women..." according to Atticus in his statements during the trial.
• The town of Maycomb has never "...seen any jury decide in favor of a colored man over a white man," according to Reverend Sykes.
• White people give coloured folks hell without stopping to think that they're people too, according to Mr. Raymond.

There are lots of other things you could say about racism in To Kill a Mockingbird, too. After brainstorming, you need to pick at least three main points and add them to your thesis statement. Your final thesis statement, which you can use in the introduction to an essay, might read: Racism is the main theme explored in To Kill a Mockingbird; most of the people in town are racists, Tom is guaranteed to lose because he is a black man accused by a white woman, and racist language is used throughout the book.