A few things in no particular order:
-You can google “college interview questions” and get some ideas of what might be asked. But be ready for unexpected questions as well.
-Be prepared to talk about yourself, your accomplishments, what you can bring to the college, why you like the college.
-Have a couple of questions ready to ask if you can (that are not answered on the website). My D would write a couple of things on index cards in case she forgot (but she didn’t).
-If you have a resume, bring it as it makes it easy for the interviewer to start a conversation. My D brought a folder with a couple of copies of her resume and the index card with questions into each interview.
-This may be obvious but be on time and neatly dressed and thank the interviewer when the interview is finished and send an email afterward
-Interviews are generally considered an expression of your interest in the school and don’t carry a lot of weight in the admissions process. As long as you are interested, polite, and can speak in coherent sentences about yourself you should be fine.
Relax and enjoy. A good interview will feel like a conversation. Use the interview as a way to learn more about the college. And if you are being interviewed by an alum, keep in mind that they are not professional interviewers.