PRIMARK INTERVIEW: How should I prepare?

So I have my first job interview tomoŕrow with primark and am looking for tips.

Firstly what would be acceptable dress as a female?
The only thing that I have that is not too casual is a nice pair of blue jeans, a white t shirt and a nice mustard jumper but should i look for something else?
How formal is too formal for a part time retail assistant interview?

Also what kind of questions am I likely to be asked and what is the best way to respond?

NB. I indicated I was available to work the Thursday night shift that in retrospect doesn't suit and was wondering when would be the best time to bring this up?