I've gone on a summer job two summers in a row, but let me tell you, the adrenaline before an upcoming interview doesn't go down, whether it's the visa interview or the one with employers.
From personal experience I can say that it's not very scary – it's just the waiting and the fear beforehand that I don't like.
For the interview itself I put on a suit – you don't have to be as formal as me, but my mom put a lot of pressure on me :D – but you still have to show interest and look presentable. I went into a big hall with other people like me, anxiously waiting to see what would happen. The interview itself was a short, friendly conversation, with questions like: tell us a bit about yourself – what you do/study, hobbies, brothers/sisters, why you decided to take part in the program, what kind of work you want to do, whether you know anything about the region you're applying for (it's good to say something)
Then it was my turn to ask whatever came to mind, but it's better to prepare a few questions in your head in advance, because I personally forgot everything I had planned to ask. So this is the moment to state your expectations and requirements, if you have more specific ones. The employer/representative gives you an offer, and you either accept or decline it. I've never seen anyone unhappy – after all, everyone has done their research in advance on where they can go, what they will work as and the pay. And so in 5 minutes the whole fuss was over, and I left with a job offer on another continent for 4 months.
The challenge was still ahead. Never mind all that – I had yet another interview to go through, but next time I'll tell you what we talked about, how I made the security guard laugh and other funny situations.