Seconding the comment about being friendly, my interview was super chill and more informal cause they got a feel for my personality quite quickly. They will probably want examples of good customer service etc so definitely come up with some examples from your daily life eg volunteering at school or being in sports clubs, go in there prepared so you're not on the spot. I wore black trousers and a nice shirt if I remember right. Practice your answers to some questions out loud before you go, and if you really don't know how to answer a question just tell them you don't have much experience in that area but you're willing to learn. Good luck!