I had an interview 2 weeks ago. I was completely terrified. The interview went great, though. I had practiced many questions, of course, but they only asked about my research, my future and if I had any questions (be sure to have some). Overall, it was a great experience, and I'm very happy it went very well.
Overall, it's good to be prepared, but the key I think it's to relax and have confidence in yourself. Being selected for an interview is a huge accomplishment, so that means you've already caused a good impression. Just act natural . Good luck!