I used to work at Argos. The interview I had was around Christmas time so they were recruiting a lot of people around that time, so I don't know if my experiences will be different to yours, but I'll tell you what it involved anyway.
It was a group interview, with about 15 of us in there. First we had to do introductions, by talking to the person next to us, then introducing them to the whole group afterwards. Then there were a series of games/tests we had to do, like put a bunch of Argos items in numerical (catalogue number) order. Then finally we had to come up with a Christmas advert for Argos and act it out in front of the others.
After that, we went into a room one on one with a manager, and they talked through our application forms and CVs with us, just asking bits and pieces about what was on it. That bit sounds quite scary, but it was quite laid back and informal.
I'm not sure they'll be recruiting enough to do the first bit, but the second bit is probably quite standard, so be ready for that; if you were a bit generous in your CV, be ready to discuss the things you wrote, although they won't grill you.
As for dress and things, I'd say you can never be too smart for an interview. If everyone came in jeans and a t-shirt, it'd look normal, but if a handful of people wear suits, they'll stand out a lot. Wear a suit if you can, and even if you're the only one there who does, you can always just say you can straight from school or another interview or something. It's easier to talk down over dressing than talk up under dressing; there's nothing wrong with making an effort.
Things to take, nothing really, although maybe having a pen at hand would be useful. If it's a group thing, you'll look organised if you need one, and if it isn't, if it's one on one, a gift would be for your interviewer to not have a pen when they need one, so you can hand them yours; that'll get you remembered.
I've jabbered long enough, PM me if you have any other specific questions and I'll do my best to answer them.
Good Luck.