To start....answer truthfully....this will get you in to a program that will fit well for both you and your program...
Second, like your personal statement should have already done....try and "subtely" highlight some of your good attributes while telling them about yourself...
Watch your interviewer for non-verbal and verbal clues as to how much or how little to say. If they interrupt to ask you a question, let them and answer their question and let the subsequent conversation flow like you would with a friend. It is a very open ended question, I got it more than once, so you should be somewhat prepared to answer it. But, be flexible each time you answer it, as the conversation will go different directions every time.
Good luck