You might want to start with a general statement about aesthetics, such as “There is no disputing about taste.” Then proceed with a broad definition of art and/or beauty, working toward your thesis statement (which you should determine by your own opinion or the preponderance of the evidence you have gathered.) As you follow the “canons” of rhetoric—gathering evidence, wording the evidence in a convincing way, etc., keep in mind your central view of aesthetics – external and objective or internal and subjective. Build your argument structure around the tried and true mechanics of good essay writing, using the best aesthetic models at your disposal. Try to avoid clichéd examples such as The Mona Lisa or David, and freshen your argument with a clear discussion of controversial models – Pollock, or rap music, or elevator music. Keep your thesis statement in mind, and end with a restatement of it in new language.