A thesis statement, located in the introduction paragraph, is a one sentence summary of your paper's argument or purpose. Most theses answer a question and include the cause and effect of an issue. The topic of your paper is the effect that you have observed and the cause is how that effect came to exist. For example, if the topic of your paper is homelessness in Sacramento, your thesis statement will end up as something like "homelessness in Sacramento is caused by..."
This video from Linda Brown at Tulsa Community College provides a brief overview of developing your thesis statement.