The first thing to remember when you begin to write is that the first draft does not need to be perfect. Count on rewriting. Your first draft may well be a mix of planning and improvising—taking you in a direction you hadn't originally intended. Even when you begin a first draft, you may make changes in your thesis statement or your organizational plan.
This guideline changes for timed writing assignments. You will probably have time for only one draft, so organization is crucial. Spend a few moments planning your answer, noting examples, and then begin writing.