No one should expect to sit in front of an interview panel unprepared and get the job.
The recruitment process usually requires that you sit in front of an interview panel. This panel generally consists of the management team responsible for the post you have applied for, a member of the company Human Resources department, and sometimes an independent who assists and observes.
The following links provide advice and guidance on what you need to do to prepare yourself, from how you dress, to practicing how to answer certain types of questions. There is plenty here for you to check out. Please do re-visit regularly to read up on the information provided here and to practice your skills.
Have a look at this site, which provides guidance on interview techniques, including advice specifically for school leavers.
An interviewer will form an impression about you as soon as you enter the interview room. This video provides a brief introduction to developing good interview skills:
It is a common misconception that you should prepare for the world of work just before you leave school. In fact, your preparation should start early on, at least from Year 9, so that you can develop your CV writing and interview skills to a point where they will come naturally to you once you enter the workplace.