Congratulations!! I had 4 interviews this past week and the main advice I have is to show your enthusiasm! Make sure they can see why you are excited about their program!! Ask good questions, you are assessing them as much as they are assessing you! I'd also have a list of prepared answers to common questions like "tell us a bit about yourself" or "why public health"! Finally, i know its really hard but try to be as relaxed as possible. They already really really like your application, so they are just checking that you can talk clearly and you fit the vibe of the department. Be yourself !!! And good luck.