Do your research on what areas your faculty member specializes in. Read their profile online and skim some of their publications to get a sense of what kinds of research they do. Be prepared to show why this research area relates to your goal, what things you want to learn from the position, and what qualifications you already have. Also ask any questions you may have about the research during the interview
Review what topics have been covered in all the courses you've taken. Don't exaggerate; if a professor is willing to sit down with you for an interview, it's probably to see what they'll need to teach you before you begin working rather than to decide if they want to hire you. Have a good idea of what about the research interests you and what you're hoping to get out of it.