Talk to your high school counselor or your college adviser. They can give you some essay examples and you can have them proofread your essay once you are done writing it these professionals will have insight into what the admissions officers are looking for in an essay. And it is always a good idea to have one or two people read over your finished essay in order to make sure there are no grammatical errors.
Many students who dont have trouble expressing themselves aloud freeze when it comes to writing an essay and as a result their writing appears tentative and unnatural. The ability to write in a natural confident "voice" is one of the most important skills that you can develop as a student and one which will pay huge dividends throughout your academic career and beyond. A good Essay Writing Coach can help you develop a comfortable consistent and natural writing style or "voice" which can be applied to any topic or subject matter. Just as in life a confident writing style can go a long way towards achieving success.
I then visited over 50 New Zealand High Schools and spoke with over 8000 kiwi kids about essay writing. These students reported exactly the same challenges as I had previously encountered and more. The result has been two books and a DVD that have helped kids achieve some of the potential that sits inside all of us.