Research your essay the topic. You do not want to formulate a thesis before you take this step because it could bias your research.
Formulate the main idea of the essay. This is not the same thing as your thesis statement. The main idea encompasses your topic, but is not refined or focused, and it is not yet in the form of an argument.
Organize your information and create an outline that will lay out how you plan to build your argument. This will force you to make important decisions about what the main points are, how they fit together and in what order you want to present your information.
Start your thesis by describing the grounds on which you are going to make your statement. Choose a strong phrase like "I hypothesize that," for an experiment or, "An analysis of [topic] reveals that," for an analytical paper.
Reveal the conclusion you have drawn. For example, "I hypothesize that ice cream sales will be higher today."
End your sentence by listing the main points, in order, as to why you came to that conclusion. Use the word because or another transition term to make the sentence flow. For example, "I hypothesize that ice cream sales will be higher today because of recent market trends, the increased temperature and an advertising campaign.