Don't be nervous. You are a qualified candidat for that position that is why you got chance for an interview. Interview is not only to question candidates. It's also time for you to find out more about school, it's program, and also about faculties. You could ask them about scholorships and teaching assistant positions too. If you prepare yourself to ask them question, then you won't be nervous. There will be two way communication which will make you feel comfortable. So you should take an interview like a very casual talk with some new people. You should look confident. So relax and enjoy your free trip. Good luck.