Don't expect too much from an essay. The application essay is important, but it's not the only thing that is considered. "Can the essay make a difference in getting the 'thin versus thick' envelope?
Ask them to tell you what they like best about your essay, and what you can do to improve it. Criticism of your writing can be tough to hear, but try to listen with an open mind. Unfortunately, some students try to list every single reason, their stellar academic record, their athletic prowess, their community service, all in a page or two. When that happens, the essay looks like a grocery list. Read
Write and rewrite. Don't try to write a masterpiece on your first try. It's not possible, and all that pressure is likely to give you writer's block. For your first draft, write anything that comes to mind about your topic. Either way, don't be afraid to make major changes at this stage. Are there details that don't really relate to the topic? Cut them. Do you need another example? Put it in. Some people are "fat" writers: they write long, wordy first drafts that need to be shortened later. Others are "skinny" writers: they write short and simple first drafts and then need to add details or examples to "flesh out" the skeleton. Start early. The more time you have, the less stress you'll have. You'll have plenty of time to give the essay your best effort. Be yourself. Take a moment to think about what interests you, what you love to talk about, what makes you sit up and take notice if it's mentioned in class or on TV.