If you're looking for a thesis statement for an essay, I would definitely check out the post above mine for some great answers.
However, if you're looking for a main idea of the novel, or a theme, there are several of them. Most of them come from the wisdom of Atticus Finch.
In Chapter Three, he explains to Scout that you can't really understand a person until you walk around in his/her skin, meaning we shouldn't judge people until we know their full story.
Going along with that, Lee tries to tell us that people aren't always what they seem. She uses Mrs. Dubose, Mr. Raymond, Boo Radley and the Missionary Circle for examples of this. Tom Robinson can also be an example because people were making judgements on him just because he was black.
In Chapter 23, when Aunt Alexandra tells Scout she is not allowed to invite Walter Cunningham over, Scout remarks that she thinks there's "one type of folks: folks." Scout doesn't see the differences in skin color and social status that seems to consume most of the adults in Maycomb.
The outcome of the Tom Robinson trial shows the tragic outcomes of bigotry, racism and close-minded thinking.
Atticus' last live of the novel, when he is putting Scout to bed, reminds the reader that most people are nice when you really see them, almost encapsulating all of the theme--Atticus takes his own advice, puts himself in other people's shoes (including Bob Ewell) and is able to see the good or reasoning behind most people's actions.
We usually don't think of works of fiction as having "thesis statements." You can make this question easier by asking yourself, "What was Harper Lee's point? What did she want to get across to people when she wrote the book?"
There is no one answer here. I would advise you to ask yourself one of the following questions:
What lesson does Atticus teach readers throughout the book?
What is the "moral" of the Tom Robinson trial?
What do we learn from Scout, Jem, and Dill about growing up?
The answer to any one of those questions would probably make a great thesis statement!