Understand the purpose of a thesis. A thesis states, in one to two sentences, the main purpose of your paper. The thesis lets a reader know what he or she can expect from your essay. It also works as a guide for the writer to keep the content of the paper focused and organized. If you begin to write about a subject that is not relevant to your thesis, that subject should not be in your paper. If your essay does not fully develop and explain your thesis, you need to keep writing.
Know the type of essay you are writing. The type of essay is reflected in the thesis statement. For example, an analytical paper breaks an issue into its parts, explaining how it works or what it is. An analytical thesis tells the topic that is being analyzed and answers a how or why question about that topic. An expository essay explains something to the reader. An expository thesis summarizes that explanation into one or two sentences. If you are writing an argumentative paper, you are taking a stand about an issue and then providing evidence in the essay to back up your viewpoint. The goal is to convince your reader that your way of thinking is the correct one. An argumentative thesis asserts your stand on the issue.
Write a preliminary thesis and make sure it is not too broad. A common mistake in writing a thesis statement is to make it too broad, such as writing a thesis that states "pollution is bad." Be specific. Focus on a type of pollution, such as industrial waste or post-consumer waste. Explain what you mean by "bad." Perhaps you would focus on how post-consumer waste is harming the world's oceans. A thesis needs to be narrow enough that you have a focus for the paper that guides the research included in the essay.
Place the thesis in the correct spot. A thesis appears at the end of a paper's introductory paragraph. If your introduction ends up being more than a single paragraph, the thesis is placed at the end of the introduction's last paragraph.