The first thing to remember when beginning to write an essay is that you don't have to create a perfect first draft. Count on rewriting. Your first draft may well be a mixture of planning and improvising — letting yourself move in a direction you hadn't originally intended.
Remember that writing is not a straightforward process, that even when you begin a first draft, you may make changes in your thesis statement or in your organizational plan.
The exception to this rule about first drafts is in timed writing assignments, for which you may have time for only one draft. Spend a few moments planning your answer, noting examples you intend to use, and then move ahead. Graders of these assignments are looking for substance, support for your points, clarity, and overall correctness; they will not penalize you for small mechanical errors or less-than-perfect organization.