A thesis statement is a sentence that expresses the main point or focus of an essay, and it usually appears as the last line of your introduction paragraph. The thesis is not a question, but rather a statement, which should match the general purpose of the assignment: for example, a statement of opinion for an argument essay, or a statement of fact for a report of information. Someone may disagree with your thesis, so you must spend the rest of your essay trying to support it with evidence and explanations of how the evidence backs it up.
An example: In Louise Erdrich's "Red Convertible," the car symbolizes Lyman and Henry's unique relationship.