[VirtusGraphics]

Like others have said, you just need to relax and not overthink. Be straightforward, but kind.
My mother is a boss, and the one in charge of hiring people. I've spoken to her and my father about what they see in an worker, what they think is important and so on.
They said that the most important thing is tha they're reliable, do what they're told and show up at work. So keep that in mind. It's not really knowledge people are looking for most of the time, as everything can be taught (thought it'd save time and money).

People do know one get nervous during an interview, so you could take the same tips as people give you for presentations.
Look them in the eyes as you speak, nod and respond, be elaborate, and don't fiddle with stuff. I usually put things that I may unconsciously start to fiddle with away, like keys, usb, phone, paper or anything in my pockets. I also take of my jacket so I don't do anything with buttons, zippers and so on. It's a good countermeassure for beginnners.

I've had a job since I was 13, and showing up and being reliable was the most important thing.