I certainly hope you know why you got to this point in your life and what made you make the choices that led you here. The overwhelming majority of candidates know this, or think they do, but a lot of them has never said it out load, had their reasons criticized and undermined, or had to ague about aspects of their decision making. A college interview basically comes down to that, think that you are talking with one of these pessimistic people that always find the fault in things, and that you need to show that person that you are the person for the job, now stop thinking and start talking!
This may sound crazy, but trust me, its not. Start talking about your reasons, about your life, what you are passionate about and many other things, but talk – really – speak out load, slowly, make small comic remarks, add a little color to your sentences and personal comments about decisions and certain periods in your life. Talk while you drive, while you take a shower, while walking to school or walking your dog, before going to sleep… while watching a boring TV show, the important thing is to let it out and listen to how you sound, how IT sounds, because this is the way its going to sound the day of your college interview. And for this you better prepare.
Practice answering tough, directed questions about your experience, motivation and goals, be prepared to draw colorations between your experience and what seems to be the need of the institution. For example, if you are applying to a course that requires strong analytical skills, you should have example ready to demonstrate your strong analytical skills. Some questions may involve topic like college admissions, be prepared for that too (research college admissions).