DRESS FOR SUCCESS! It's casual, but they want you to shine, that's all they care about.
Beware of the ads stating "fun" atmosphere - It's not. They are fishing for you because it's a scam for free work after 1 month. Be sure to ask them about the turnover rate and don't buy that they employee pool is poor, they've hired people from over 100 miles away - they are the scavenger.