Determine what type of essay you are writing. Are you writing an analytical paper in which you break down and evaluate an issue or idea, an expository paper in which you explain something to your reader, or an argumentative paper in which you make claims about something and give evidence for those claims?
Brainstorm your thesis statement. Usually, a thesis statement is an answer to a question. A good thesis statement is also the main idea of the whole essay. So, what is the question that needs to be answered? Once you have the question, write down possible answers to that question.
From the list of answers you've brainstormed, pick one answer or idea that you feel you can discuss best in your essay. Make sure you have enough evidence to support this central idea in your essay. Keep in mind, a thesis statement should be specific and shouldn't be any longer than one sentence. It should also express the purpose of your essay.
Decide why that idea best answers the question. Usually good thesis statements include three reasons why the idea answers the question, though you can include two or more reasons depending on how much evidence you have and how many paragraphs you plan to write. (Example: Dogs are the best pets in a household because reason x, reason y, and reason z.)
If it's hard for you to write a thesis statement right away, consider writing the first draft of the essay first and then work on your thesis statement once you're done with the draft, since this way you already know what evidence you will be providing and you have a better understanding of how the essay will unfold.
Formulate a thesis that someone could argue against. Take a stand, and stick to it throughout the article. Avoid stating the obvious or writing a thesis statement that can't be disputed.