A thesis statement should be specific, explaining what you will cover, and should relate to the evidence you will present in the remainder of the paper. Traditionally, a thesis statement should be the final sentence in the introductory paragraph of the paper.
You should limit your thesis statement to the scope of your paper, and should use it to present a specific thought or argument. It can be as simple as stating your argument and a summary of your supporting facts. For example: "The Mayans were more powerful than the Incas because they developed tools sooner, had a more advanced educational system and valued military forces."