Read the essay prompt carefully at least once, making sure that you understand every part of the assignment.
Reword the prompt in the form of a question. Some AP essay prompts are questions, but the ones that are not imply questions. Figure out what the prompt is really asking. If a prompt asks you to "argue for or against single-gender classrooms," for example, reword it to say, "Do single-gender classrooms help or harm education?"
Answer the question definitively in one complete sentence to make your thesis statement. Instead of writing that single-gender classrooms are good or bad, for example, write, "Single gender classrooms make it easier for students to learn."
Ask questions to test your thesis statement: Is the thesis statement debatable? Is the thesis provable or does it reach too far? If you write, "Single gender classrooms separate the male from the female students," for example, you do not have a debatable thesis statement; if you write, "Single gender classrooms will cure every educational problem in the U.S.," you do not have a provable thesis statement.