I have, in the past, been an interviewer for retail, but more in cosmetics than clothing so not sure if this will help especially, hope so though.
They will probably ask why you want to work there and what you know about the brand or the various groups they are most keen to market to. They might ask something about what you think the colours or popular sellers will be for Summer, for example.
They may ask about your flexibility in terms of hours (not sure whether this is as it used to be as possibly contracts are either zero/low hours and very flexible, or hardly at all, sadly). However, be prepared to work intensively with not much choice around Christmas/Boxing Day/New Year as this is THE busy time and also prime stocktake time as well. This is also, in a roundabout way, a teamwork question.
They will probably ask you about times when you have given what you considered good customer service and what the outcome was (don't forget to give that even if they don't ask).
They may ask you about how you would prioritise certain things on the shop floor if they landed all at once (e.g. a customer complaint, a queue, the phone ringing, a big delivery arriving and needing dealing with, and a display falling on the floor, for example). They might also ask if you have any cashing-up experience and what you would do if you had an end-of-day total that did not balance.
Have some questions ready for them at the end too. Ask what is their most fulfilling part of working there. Perhaps what their ideal development would be for the next 12 months.
If you get stuck, a good time-buyer is "That's a good question..." I think being enthusiastic and good eye contact etc will give a good impression as regards possible customer rapport. What to wear - not sure. Something smart and in line with their style if you can.
Good luck.