Know the sneaker society and keep up to date with fashion
Polo and a nice pants and running shoes as they will be looking at the type of shoes you wear
Just be talkative because you're trying too convince not only the boss but yourself that you are prepared for this job
Just be confident in yourself and have a positive attitude.
You should prepare as being ready to sell something to the person interviewing you.
Since it's not a formal job, you can wear a clean pair of sneakers, but still dress tastefully. They will also ask a lot of questions about sales approaches teamwork since that is the environment of the workplace.