Your
thesis statement is the main point of your essay. It is essentially one sentence that says what the essay is about. For example, your thesis statement might be "Dogs are descended from wolves." You can then use this as the basic premise to write your entire essay, remembering that all of the different points throughout need to lead back to this one main thesis. You should usually state your thesis in your introductory paragraph.
The thesis statement should be broad enough that you have enough to say about it, but not so broad that you can't be thorough.