First and foremost be professional. Yes you are interviewing for a job where you get to where shorts and a shirt, but take the time to present yourself in a professional way. As far as the interview goes, have a solid understanding of program design and exercise in regards to the general population. The majority of the clients you will work with are not superstar athletes, just people who want to look and feel better. So be able to explain how you will be able to program for someone like this. Finally, don't be afraid to talk about what you will find challenging about this job. You will have to "sell" personal training, so if you are uncomfortable with this, make sure to express that so that you can begin to get help right away.
Emphasize your experience in sales and the ability to persuade potential clients to pay you train with them. Your degree in Exercise Science or a related program is helpful too, but the biggest component of personal training is commission-based sales.