Or indeed already have one...
What would be your USP? And what do you think would be your major threat?
My USP would be British made. It wouldnt be high fashion but you know you could go there and get a solid, reliable white t-shirt that wasnt see through and wouldnt sag and had french seams.
Obiously, that is expensive so the 2 main threats would be to get people to buy there to begin with and then to come back when they have already bought my perfect t-shirt so dont need to replace it for 3 years.