During my shopping experience I have found they will provide you with a pair of shoes, and even a bra (but this is gross so I would wear my own). Wear something comfortable so that you don't have a difficult time gettting undressed and dressed.
As far as what to expect, I think it was like buying a car. You walk in and say you are just looking, and they will bagger you anyway they know to try and get you to buy. They will ask you your budget then show you dresses that are way out of it. Then when you are "test driving" the dress they will put your hair up and stick a veil on you and try to sell you on the whole picture. If a store makes you feel uncomfortable when you are just looking, then go someplace else. You will be dealing with them for several months between shopping and fittings and you don't need the headache.
I know this sounds negative, but weddings are a big business. Don't get me started on Davids Bridal. Well just one word of caution with Davids. Actually two. It's nice to be able to try dresses on that are closer to your size, since places only stock size 12 and for most that won't fit. But when you order a dress they will tell you to order a size larger then you really need because then you will need more alterations (ie. it will cost more $$$). Ask to see a size chart and you should purchase the dress that fits the largest of your measurments. Also do not by any means get your alterations done at Davids. They are a fixed price. Their prices are outrageous. You can get the same alterations done at any shop. Ask around to find a reputable one.
I know I know I sound negative. I just think you need to shop smartly. The dress is a big deal. Don't go in blindly shopping. Enjoy the whole experience. You only get to be the center of the day once!
Wedding October 11th, 2008. Bridesmaid too many times.