Hi, It's great that you are looking for a job at 16, that shows allot about you as a person already, all good(:
Too answer your question; Sorry but DON'T go there wearing a floral dress.
This is what you should wear; something that says; responsible, mature and together. Like pin striped black dress pants with a white blouse, you could wear either black flats or black pumps but I would go with the pumps. Target is a good place too go, and there not too expensive. It wouldn't even be a bad idea to wear a fitted blazer over the blouse, you should dress too impress. And if you show up in all black, "like your working there" so you put it. That doesn't matter, they dress there employees on what they like them too wear, and anyways they don't know that you wear flowery dresses(: unless you show up in one, haha LIKe i said i would go with blaCK PINstriped dress pants, a white dressy shirt or blouse and some black pumps..You can NEVER go wrong with dressing that way it always looks good and on everyone(: Hope you get the job(: