There are many different formats for interviews, which take place virtually, by telephone, or in person. Essentially you are in a great position - the university or employer is interested in you and wants to know more!
University/employer interviewers will be looking for:
motivation and enthusiasm
good oral communication and language skills
a variety of different skills relevant to the subject/course/job applied for: knowledge, interpersonal, teamwork, flexibility, adaptability
a positive attitude to dealing with problems and difficulties
commitment to the subject/course/job applied for
evidence of your ability to use your initiative and be proactive
They will also be looking at:
how you go about answering the questions
your attitude to problems and challenges
whether you'll 'fit' within the course/department/institution/organisation
your motivation and self-awareness
The Prospects website contains 'Interview tips', with information on how to prepare, and some typical questions. Unifrog also has lots of information and advice on interviews.
This Barclays Lifeskills website is excellent for practicing your interview skills. Link is here. Also you can do practice interviews here.