The key is to be as prepared as possible; it is always a great tactic to practise answers to possible questions, this way you are less likely to freeze up when being asked them for real. It is also useful to do some background research on the company; if you can recall some recent successes or milestones in the company history this will show the interviewer you are keen.
Taking the example of an interview for a call centre position can help to illustrate the general kind of questions you can expect. For such a position, it is very likely they will ask a question such as 'how would you deal with an irate customer who has been put through to you?' A good answer to this would be that you would continue to talk to the angry customer respectfully, maintaining a calm disposition whilst assuring them that you will do your utmost to solve their problem.
Another relevant question may be 'why do you think you would be ideal for a role in the call centre?' This is an opportunity to show off your credentials. A strong answer to this question is to list all your strengths and make them seem relevant and adaptable to the role. Say you are great with people and can strike up an instant rapport; even over the phone. Always say you are hard working, dedicated and eager to learn.
If you practise your answers until you are confident, this should give you great assurance when going in to the interview. Try and let this confidence show without being cocky or arrogant.