The thesis statement in an essay explains the point of the paper for the reader. All essays need a thesis to give readers information about the essay's purpose. Part of what makes a thesis effective is how well it fits the assignment, since one that is too broad may make the point difficult for the reader to understand.
The language you choose for the thesis affects how broad it sounds. Your thesis should reflect your opinion about your topic rather than a general belief. For instance, "Some people believe obesity is a problem for children" is less effective than, "Childhood obesity is a problem." You can also create a narrower thesis by using words that focus your point to a particular group by age, gender, geography or something similar. For example, the obesity idea can be further narrowed, "Childhood obesity is a problem in the U.S." Know what your essay's purpose is and then select strong specific words to illustrate that purpose.
A persuasive or argument essay should have a thesis that makes a claim you can prove within the body of the paper. That claim makes a statement of opinion, setting up how you will prove that opinion and get the readers to change their belief or perform an action. Persuasive and argument essays may have a thesis like, "In order to reduce childhood obesity in the U.S., the government must implement educational programs, school incentives and dietary restrictions."