When you are applying to medical or dental school, you need appropriate undergraduate coursework with good grades, and good scores on the MCAT if the schools to which you are applying require it. However, there are far more qualified applicants than there are places, especially in the strongest programs. Thus the personal statement can be a significant factor in whether you are admitted.
In the personal statement, you should show that you are the type of person who could succeed in medical school and in a career as a medical professional. This means more than just being smart. First, you should give the readers a sense of why you want to pursue medicine and how these motivations will make you a good doctor. Avoid vague generalizations about "wanting to help people" and think about specific issues or experiences.
You also want to impress upon your readers that you are mature, reliable, highly motivated, and able to persevere in doing difficult tasks. Also, you should show that you are capable of genuine concern for your future patients, rather than being self-centered.
Finally, you should demonstrate that you are careful and detail oriented by proofreading carefully for grammar and spelling.