Other than taking a couple minutes to detail your undergrad career, ECs, why you want to be a doctor, etc. I think the following advice is very helpful:
- How you articulate your answer to this question will tell the interviewer as much, if not even more, about you than what you actually say. And I'm not talking about using giant, weird vocabulary to show that you are "smart", I'm talking about making sure you come across as well-intentioned, mature, friendly, and able to formulate and express your thoughts logically and concisely. Be genuine. Don't sound like you wrote out and memorized a canned response before you came in.
- Stay on target. This is not the time to tell your whole life story, but only your life story as it pertains to your decision to become a doctor and why you'll be a good one. If you are the oldest of 10 kids don't just say that you are the oldest of 10 kids, but find a way to subtly show how that experience has helped you develop admirable traits.
- Don't talk to much. I imagine that most people should be able to effectively answer this question in 3 minutes tops. Since this is often the first question you will be asked, it will set the stage for the rest of the interview. If you are smart about it, you can sort of direct the interview this way. If you ramble about everything you've ever done, you'll leave no room for follow-up questions from the interviewer and then he or she will move on to their "back-up" questions just to fill time and it won't be as productive of an interview. Instead of doing that, just restrain yourself and give powerful, succinct bullet points about all your activities/events and their impact on you, but leave out most of the details. That way the interviewer will can choose what they are most interested in and say, "so tell me more about ____". THEN you can expound in greater detail. That way you spend your time talking about the things that the interviewer thinks are most likely to be of importance to gaining admissions.