Try and find a niche. Who would be the best buyers of your shirts?
Here's a guy
that built his shirt business by understanding exactly who would be the ideal buyers of his shirts, and then focused on marketing/selling only to those buyers.
My wife and I have a website that sells paint, which is too vague of a description. But the paint we have is a specialty paint you can use to paint your furniture and home decor items. So, we focus on targeting women who are into 'crafts, hobby, DIY etc'. Once you know who your target market is, it makes it easier to segment markets and find those people. We find our ideal customer on Facebook and through bloggers who blog about DIY and craft stuff.
So try and figure out exactly who your shirts are for:
athletes? skaters? women? men? kids? babies? business? casual?
Then craft a marketing message and sales pitch that would appeal to your buyer and then deliver that message wherever your buyers are likely to be found. One tip is to go find the person who already is selling something to your ideal market and then sell them on why they should either carry your shirts, or introduce you to their group of customers/followers.