Determine whether the topic is broad or narrow. If you've been assigned a historical topic, for example, narrow it down to a part of history, like a specific event or a person. The more specific the topic, the easier it will be to define your thesis statement.
Determine the type of essay you plan to write. Are you trying to defend an argument? Is your essay intended to make a decision on a current discussion? What are you trying to tell your reader? Choose what you want your essay to accomplish before defining the thesis.
Write a thesis statement that presents a single idea about a controversial topic, explores the issue and gives a definite position about it.
Narrow down your subject.For example, if the assignment has to do with pollution, start with something like the ocean and then expand upon your subtopic by conducting research and discovering facts surrounding ocean pollution. Incorporate a statistic that relates to your thesis and elaborate on the points that you will consider in your essay.