A thesis statement introduces the topic of your paper and states your opinion on the topic. A good thesis statement will also encapsulate the argument of an essay by describing a problem you want to solve or a question you want to answer. It's usually one or two sentences long—sometimes three, if your argument or topic is complex or contentious—but not more than that. Most writers present their thesis statement near the beginning of a paper.
Here's an example. "Existing aerospace technology is insufficient to support a return journey from Mars. Vehicle thrust systems, and life support systems, cannot be maintained over a period of years without prohibitive strains on a vehicle's payload or the cost of the mission." Notice how this statement tells you that the following essay will be about the obstacles to getting to Mars and back. It implies that the journey is desirable and that I think the barriers are not all technological. It also gives you an idea of how my argument will look: I'll focus on the cost and on vehicle payload.
Try the same thing about obesity. First, ask yourself what you think about obesity. Is it a problem? Why do you think that? What evidence can you show in your essay to support your opinion? If you can answer those questions, you'll be on your way toward crafting your thesis statement. The trick then will be to condense what you know into a few sentences. Don't try to tell your whole story in those sentences. Just give readers a hint, a brief look, or a summary. Good luck!