if it's a group interview you have to engage in the group like you are a natural talker kind of person, so if you're sitting there too silent or talking too much and not letting others speak they probably aren't going to hire you. So be natural, just like a normal conversation and listen to others speak and you speak or make a new topic etc - don't be too fake because you are being watched and they'll probably know if you sound a bit fake. And of course when you speak try to not go too far off topic because then it shows you don't know how to interest and engage and listen to people.
if the dress code is 'smart casual' dress up a little bit to look kind of clean, presentable/neat/modest, e.g. a white shirt with a black skirt, or a smart looking casual ish dress with a cardigan. I don't think you should wear jeans to a smart casual thing, but black work pants with a white shirt is okay for smart casual. Idk what to wear if I was a guy, but I wouldn't really imagine wearing shorts maybe. And for shoes, no thongs, but I don't think you should be wearing really formal business attire shoes either to this interview as it is unnecessary
Don't go to the point that you have a full business suit on kind of thing lol because going all out when it's only smart casual or your typical retail job isn't going to say to the employers 'yes I'll employ you' - I think they kind of see it as a little vain, and it's not an advantage over someone who's wearing normal clothes or clothes that look like they went to the beach.
but as alphabetafeta96 said, show like you are interested in the job by asking questions and appear interested throughout the whole interview as well. Get some decent sleep the night before, don't appear tired in the interview (e.g. unhappy or :| face, droopy eyes, too pale, being really quiet, dozing off, looking down instead of at others, looking at the wall or ceiling or floor, or completely not paying attention lol) and set a good impression, be friendly and seem outgoing. Don't appear as if you're eager for the interview to end esp at the end by being suspiciously happy and suddenly more awake at certain parts. Be calm, happy, smile and be natural throughout. And prepare possible answers to questions beforehand too. And most importantly don't be late because nobody likes latecomers. Also learn about the company that's interviewing you and what products they sell and how you can help.