Not everyone can wear cool tshirts that can truly be called cool. That's why you need to find cool tshirts from a website that understands what it means to be cool.
First of all, you won't find the coolest T-shirts at the mall or at a regular store. Those places have the dull t-shirts that everyone else has been wearing for months. That's not cool. You need to find a source for cool T-shirts that features designs that no one else has seen yet. You need a find a website for cool tshirts that emails you the second a new design is available. You just can't get that sort of service from a major department store.
You should also find a place that isn't afraid to be controversial or offensive. Being cool means not being worried about what other people think. At the same time, being cool means knowing the difference between being funny, and just being obnoxious and crude. Remember, being cool isn't for everyone. It takes wit and good taste and vision.