It depends on the stores and the company you are doing demos for, but usually you should own:
A white button up shirt, black pants, a black apron, and comfortable close-toed black shoes (no athletic shoes for most demo companies). This is standard demo attire everywhere I do demos except for at the natural food stores, which are much more lenient and casual about your attire, as long as you have clean clothing and look tidy they usually aren't that picky.
Your own table and table cloth, and sometimes your own waste can and trash bags
Most companies I do demos for either provide products or I purchase the necessary items and they reimburse.
I am not allowed to sit down, drink, or eat during any of my demos. I never asked about bathroom breaks, but I don't take them because I don't feel comfortable leaving my demo table unattended, as sometimes I have a hot crockpot or other things on my table I don't want people to touch. Once I walked a few feet away from my table to point out an item to a customer and when I turned around, some people were helping themselves to the food in my crockpot, and since I have food handling gloves on I can't stand it when people touch the containers, cups, spoons, and other items on my table (who knows where their hands have been). And a little kid could burn their hands on the outside of my crockpot (lots of people don't watch their children properly and let them run around the store getting into stuff). I just go to the restroom before I start the demo and when I get done with the demo and then I don't have to worry about it.
The stores where I do the demos allow me to pick and choose where I want to set up, but as eyelove2shop wrote, it's usually very close to the product.
The nutritional company I work for sends me tons of product to use for demos and for my personal use. I make $25 per hour for all of the companies I work for. I know there are some companies that pay more. I am too old to do the types of events that eyelove2shop mentioned (private events and promotional modeling type stuff).