Use your personal statement to tell the admissions committee members why you want to study at their school. Tell them why you want to study the subject you are applying for and why their program is the one you want to participate in. Avoid saying "I want to be a doctor because ..." Instead, tell the committee a story about why you have chosen this particular course of study. Be sure to research the school's program so you can point out concrete reasons you think it is the best program for you.
Once you have told the admissions committee why you want to attend the program, you should tell committee members why you will be successful in the program and in your chosen profession. Again, avoid saying, "I will be a great doctor because ..." Instead, tell them an anecdote about skills you have that will make you great at your chosen field of study. The admissions committee is charged with finding people that will be successful in the program and will represent the program well in the profession upon graduation. Use your personal statement to emphasize your aptitude for the career you want to pursue.
You should also tell the admissions committee what you will bring to the program. Highlight any experiences you have in the field, any leadership experience you may have, and any other information that will show the committee that you will be an asset to the school and the program.