Look up a list of common interview questions (https://www.thebalance.com/top-interview-questions-and-best-answers-2061225) and come up with an answer for each of them. In my experience, anecdotes make the best answers to “what is your greatest strength?”-type questions. Ask a parent or a friend to ask you the questions so you can practice your answers. Learn exactly what the focus of the program you’re applying to is, and if you know who’s going to be interviewing you, look them up too to see what their main professional interest is. You don’t need to boast about your intelligence or qualifications; that’s what a resume is for. Your job is to convince the interviewer that you’re a person they want working in their lab, and your interest and people skills will help you there.