nothing too fancy (its a supermarket) but mainly you want to look like you want to work, and not some little rich boy that is being forced to work(unless you happen to be one). a collared shirt and a decent pair of pants(trousers??)
French Knickers
Pleated khakis, a button down, collared shirt, and a pair of loafers. Take a copy of your resume even if it says very little on it. Shave, be 10 minutes early, say "yes sir" and the job is yours.