The first step in constructing a good thesis is to choose a broad topic or theme that you'd like to examine within the book. You'll want to be careful to work with a topic or theme for which you can provide an abundance of textual support, and readers notice different things about a book as they read. This is one of the reasons reading is such a personal experience.
Students also often find it helpful to construct a thesis that easily lends itself to the body paragraphs which will follow. If your teacher, for example, is asking for a five-paragraph essay, you may want to build three key points into your thesis.
One of the themes in the book centers around the cruelty of humans. This has various effects on Elie and others as they fight to survive. A thesis could look something like this:
Enduring ongoing cruelty can make people feel hopeless, can cause them to inflict cruelty on others, and can make them question their religion.
Your three body paragraphs would then support each of those three key ideas, focusing on various evidence throughout the work.
Another theme of the novel is the unpredictability of human nature. Remember that when the book opens, Moishe the Beadle tried to warn everyone that they were in great danger; the townspeople chose to ignore him. The soldiers inflict great pain on other humans without any real reason for their malice; they simply follow orders and in ways that continually shocks those who are imprisoned. Both the son of the rabbi and Elie turn their backs on their fathers near the end and after they have endured great trials together. A thesis for this topic could look like this:
Human nature is often unpredictable, leading people to sometimes engage with others in shocking ways. This unreliable side of the human experience is portrayed through the way others ignore Moishe the Beadle, the mindless cruelty of the soldiers in the camp, and the eventual way both the rabbi's son and Elie turn their backs on their fathers near the end of the book.