I'd make sure its on a personal level. Don't write about anything cliche like "I want to help the world". Relate it to your personal life and what, if anything drove you to wanting to be a dentist. I have a unique circumstance that I am writing about, and you should also have a reason as to why you want to be dealing with teeth. did you ever experience something that drove you to this? What is important to you? Get PERSONAL in this letter, it reflects on your desires and helps you stand out from the other "sheep".
As for format, there really isn't one that's necessary, but a good Idea would be to have an introduction, a body (perhaps a couple paragraphs) and a conclusion.