i agree with the other posters here. first, BE YOURSELF AND BE HONEST. DONT MAKE ANYTHING UP TO SOUND GOOD. BE YOURSELF! second, just tell them about yourself as the question asks: where you are from, what school you went to, your family, your friends/significant other, what you like to do for fun (if its a sport or something like that, relate this to the location of the school, for ex talk about the mountains around the area if you like to bike or hike or ski, etc), what you are passionate about, your goals (once you become a doctor, have a family), special experiences (medical mission, publication, extensive volunteering, a specific passion about a certain community or topic---underserved pop, minorities, the environment, teaching etc.. and finally, set it up so that the interviewer has a lot of things to follow up with you on. so what you can do is make a list of 3-5 things you want to get across during the interview that day. then set up your answer so as to mention those briefly and so that it sounds its very important to you. the rest of the interview will prolly center around those 3-5 things.
GOOD LUCK. AND AGAIN, I REPEAT, BE YOURSELF!!! LET YOUR ANSWER FLOW NATURALLY. DONT MEMORIZE SOMETHING. BE YOURSELF.
also, i really like lotannas answer above.