Do take your time on the essay. Although it is not long compared to some papers you have written by now, your essay must possess a depth of self-understanding that your high school research papers probably do not.
Do plan ahead. Diving right into writing the essay may be okay for some, but most students need time to brainstorm, to consider a few topics before committing to one, and to have a sense of the structure before actually beginning the writing process. Be sure to give yourself enough time to write a meaningful essay. One month's time is not unreasonable.
Do plan on editing and revising. Do it yourself, but feel free to ask a friend who loves writing or English to read your essay and provide feedback. As a matter of fact, it's best to ask a few different people. Everyone has an opinion, but if you hear the same critique from a number of individuals, then you should probably take their advice into consideration.
Do have fun with it! The essay should be a personal journey of getting to know yourself better. This process should not only be revealing to a college admissions representative, but to you as well. We write to discover, and the college essay should be a lens through which you can see yourself a bit more clearly.
Don't fill your essay with every single personal accomplishment. You want to highlight yourself without sounding like a braggart.
Don't brag about your GPA or your standardized test scores. Your application already has space for that. Actually, you should not write about anything that already appears in detail somewhere else in your application. There is no need for redundancy.
Don't rely on your parents or your best friend to be the only people who provide feedback on your essay. They know you so well that they can "read between the lines," that is, they can infer what is not written from reading the words on the page. They already know your life story, or much of it at least. A college admissions representative does not have that luxury. Be sure to spell out in clear language what you are trying to say.