The kind of thesis statement you create will depend on the nature of the assignment you have. If you have been asked to write an essay about the book on a topic of your own choosing, then your first step is to narrow your focus. The book has many ideas and characters, so the whole book is simply too big a subject for an essay, in my opinion. Even if this is a kind of book report, no one can talk about an entire book in a school paper!
So, first let's talk about ways to narrow your focus if you have a choice on what to write about . The most obvious elements in a book are its plot, characters, and setting. Is there anything you would like to talk about in one of those categories? For example, what kind of father is Atticus? How do Scout and Jem approach things differently? How does the author make Boo so mysterious to us? What event in the story shows you the most about prejudice in the town? Now, once you are able to think of a question like this, you can pursue that question in your essay. Your answer is your thesis statement. For example, you might say, "Atticus is a good father because..." Or "The author makes Boo Radley mysterious by...." A thesis statement should give your audience the subject you are writing about and your own opinion on that subject.
If you are writing a book report kind of paper, you will still need to choose a few elements of the story to focus on. What did you like about the book? What didn't you like about the book? Who was your favorite character? (When I was young, my favorite character was Scout. Now that I am older and have my own children, my favorite is Atticus.) Did the book help you to understand the way life used to be for African-Americans? Did the book help you understand why we needed civil rights legislation? All of these are questions that you might use to create a thesis statement about the book.
If we have more specifics about your assignment, we can probably help you with even more ideas.
The easiest way I can think to explain how to come up with a basic thesis statement is as follows: A thesis is a subject + an opinion. The subject has to be either a noun or a pronoun, and the opinion can be whatever you are trying to argue. For example, if you were to write a paper on school uniforms, your thesis could be something like, "School uniforms (subject) are unnecessary (opinion)." If you want to make it easier on yourself for a five-paragraph essay, you can add three opinions instead of one - "School uniforms (subject) are unnecessary (opinion), unattractive (opinion) and unoriginal (opinion)." Each body paragraph would be about each opinion in that order. As for Mockingbird, you take any character's name (subject) and then add the opinion. Atticus (subject) is a good father (opinion). Just make sure your opinion is something you can write an entire essay about. Good luck!