Appear confident ! It can seem a scary thing, all those faces looking at you, but if you prepare you'll be okay. Look at the questions you'll be expected to go over, practise on a friend beforehand to get a feel for explaning things to others, and if you're unsure of something don't try and bullshit. Say you don't know but you'll find out and either email them or tell them next time. Let us know how it goes !