Most schools invite only a small number of applicants for interviews. So, if you receive an invitation - congratulations! That is a big hurdle, and you cleared it.
Interviews vary in format - some interviewers are one-on-one, whereas other interviews may have multiple applicants or multiple interviewers.
How to prepare
Make sure you have stories that show why and how you are interested in a career in health care. The strongest answers illustrate these interests with specific examples from your personal experience. You should aim to show why you are interested in this career, and not just say that you have these interests. The weakest answers merely state the obvious: that you care about people, want to help, and find science fascinating. Everybody interested in health care has these characteristics. Admissions committees want to know what makes you stand out from the rest.
Be prepared to talk about anything and everything in your application materials. If you did research, know the hypothesis, the big picture implications, and the techniques you used. If you volunteered for an organization, know the mission of that organization. If you had a leadership role on campus, be prepared to discuss your accomplishments. If you had academic or other challenges, be ready to discuss these in an honest way.