Interviews aren’t weighted very heavily in the admissions process compared to other factors, so it’s nothing to get nervous about. You’ll also do better if you’re not nervous, so it’s best to relax as much as possible. If it helps, think of it more as a conversation than an interview, and remember that you’re allowed to ask questions too. If there are things about the school you’ve been wondering about that you can’t find out from looking on the website, talking with an alum can be a great way to get an insider’s perspective, especially if your interviewer is a more recent alum. As for preparedness, just make sure you’re prepared to talk about your interests, both academic and extracurriculars, and articulate why these things interest you. The idea of an interview is to add a more “human” perspective, so that the admissions committee can get a better sense of you as a person than they could get by just looking at your stats on paper. If there are things about you that you think are important but may not have come through very strongly on your application then the interview is a good time to focus on them.