Most college interviewers aren't trying to test you; they're really just trying to get to know you better. If you're ready to give *good answers to the following five questions, you will almost certainly be prepared for just about anything you're asked.
1. Why are you applying to this school?
2. What's your favorite subject, and do you intend to pursue this in college?
3. What do you enjoy doing when you're not in class?
4. What are three interesting things about yourself that I wouldn't know from your application?
5. What's an example of a mistake you made, a failure you endured, something you aren't good at, or anything else that you probably wouldn't bring up unless somebody asked you about it?
Your motivations for college, your intellectual interests, your interests outside of class, your personality, your level of humility and self awareness, those are the kinds of things interviewers want to get a sense of. Be comfortable discussing them and you'll probably have a great interview.
*A good answer is one that's honest, that's not contrived to sound impressive, that reveals something about yourself and that has a personal anecdote to back it up. You're just trying to help them get to know you better. You're not weaving a tall tale.