A couple of ideas:
-talk to your pre-med advisor/committee, if you have one
-buy a book on the subject, there's plenty
-be honest
-make yourself stick out. if you look at your essay after you've written it and can imagine someone else could have written it, tear it up. unique is key.
-I would avoid writing in all caps. adcoms tend to look down on that.
Be personal, but not mushy. Avoid talking about being in a respectable position, money, and your Fisher Price Doctor's Kit that you had when you were little. Tell why you want to be a doctor now and provide personal experiences that will affirm your conviction.