The Giver has some pretty broad themes, but the events within the story can help you build those up into a great, specific thesis statement with well-formed points of evidence backing it up.
Let's create an example. One theme from the book is pain/suffering. The world of The Giver deals with pain by isolating it — Jonas's society is "utopian" because no one has to deal with pain (which we actually learn makes it dystopian).
We here in our unregulated world know pain is not pleasant, but we also know pain is a human experience, and is part of growing.
Throughout the course of the story, as Jonas is given memories and learns more dark truths about the society he lives in, he begins to understand people cannot live completely regulated lives free from pain. He leaves to go Elsewhere, along with Gabriel, so they might have a chance to live unregulated lives. He hopes his leaving will transfer his memories to the people of the community, causing them to gain insight into the shallow and horrible lives they lead.
So, with all this in mind, a thesis statement relating to the theme of pain/suffering could be:
Lowry uses memory and Jonas's experiences in The Giver to show pain is part of the human experience and is necessary for people to learn, grow, and live full lives.
Just take any theme from the book that you'd like to write about, see what the book specifically says about that theme, and voila! You've got a thesis statement.