A thesis is an analysis of an idea in a book or story. To write a thesis statement, one must simply ask oneself a question about the idea that you choose, and then answer it.
The first part of writing a thesis is to pick an idea, a pattern, or theme from the ones explored in the book. For example, I will use the theme of freedom in Jules Verne's book “Twenty Thousand Leagues Under the Seas”.
The second part is to ask yourself what the writer wants to say about the idea that you're analyzing. For example, we can ask ourselves “What does Jules Verne have to say about the idea of freedom in Twenty Thousand Leagues Under the Seas”.
The next step is (obviously) to answer the question we asked in part 2. For example, we can say that Jules Verne thinks that freedom is essential. Ned and Captain Nemo, two main characters in the book, seem to think so, and we can get that the author wants to tell us, the readers, that it is.
The final step is to write the answer to your initial question in a proper paragraph. For example:
In his book, “Twenty Thousand Leagues Under the Seas”, Jules Verne uses the words and actions of characters such as Ned Land and Captain Nemo to convey the idea that freedom is essential and that people can be desperate for it. Being a science-fiction novel, the author also explores the idea of using technology to pursue freedom.