the dressing code really depends on the organization you are working in. think if no one wearing shirts in your company, how strange you will looks like if you wear. Generally, if you dun know what to wear on the first day, you can either a shirt without tie(under dress is better than over dress). then watch around, see how everyone dress. then you will have the answer. Better not wearing short sleeve shirt. very strange, if you want to wear short sleeve, consider polo T-shirt.

one thing to mention, polo T-shirt is not T-shirt. anything has collar will not consider a T-shirt.

:) have a good IA