The first rule for wearing casual clothes is to make sure you are dressed for the perfect opportunity. T-shirts match with plain pants. Wearing a relatively plain shirt top allows you to really wear whatever shorts you want. T-shirts are casual clothes that are suitable for your street style in the summer.
Stylish casual clothes are not much different from what is usually described as a casual business, but have a little more informal. A shirt under a buttonless suit will offer your clothes a completely new dimension. Using sneakers you will look cool.