From Purdue OWL:
A thesis statement is a sentence that summarizes the main point of your essay and previews your supporting points. The thesis statement is important because it guides your readers from the beginning of your essay by telling them the main idea and supporting points of your essay.
An argumentative or persuasive piece of writing must begin with a debatable thesis or claim. In other words, the thesis must be something that people could reasonably have differing opinions on. If your thesis is something that is generally agreed upon or accepted as fact then there is no reason to try to persuade people.
In short, a strong thesis statement should be a focused argument that can be disputed.
1. Determine what kind of paper you are writing:
An analytical paper breaks down an issue or an idea into its component parts, evaluates the issue or idea, and presents this breakdown and evaluation to the audience.
An expository (explanatory) paper explains something to the audience.
An argumentative paper makes a claim about a topic and justifies this claim with specific evidence. The claim could be an opinion, a policy proposal, an evaluation, a cause-and-effect statement, or an interpretation. The goal of the argumentative paper is to convince the audience that the claim is true based on the evidence provided.
If you are writing a text that does not fall under these three categories (e.g., a narrative), a thesis statement somewhere in the first paragraph could still be helpful to your reader.
2. Your thesis statement should be specific—it should cover only what you will discuss in your paper and should be supported with specific evidence.
3. The thesis statement usually appears at the end of the first paragraph of a paper (the introduction).
4. Your topic may change as you write, so you may need to revise your thesis statement to reflect exactly what you have discussed in the paper.
Examples of underdeveloped thesis statements that are broad and not debatable:
Examples of good statements that are focused and encourage debate: