I would suggest some form of comfortable casual but smart shirt, as to wearing a coat/jacket with it, you don't have to. Although, it is a good idea to carry one it the weather is not the best (visual thing, you don't need to wear it). Jeans are probably ok because you're young.
As to the interview itself, finding out regularly asked questions as regards this job helps. Then asking someone to ask you these questions in a kinda scary voice can help because it may prep you for them being scary. Practice looking them in the eye and firm handshakes (stronger with males than females). Bringing water can also help because it can give you a few seconds to think of how to answer a question you didn't prep for but practice drinking casually as well. Deep long breathing can help calm you down before the interview but be careful about it during the interview because it can look like panic sometimes.
Oh, and something else if you have time to, write true positive things down about yourself because if you can see your best attributes they probably will too.
_________________
I'm female but I have a boyfriend.
PM's welcome.