It really boils down to capturing the attention of your target audience. Don't waste money on irrelevant people. For instance, let's say hypothetically, you create a shirt with an 8 ball on it. You migtht target people who enjoy billiards.
Then think about income demographics of the various locales you're marketing to. How frequently do you think a person making $100k-$250k per year wears t-shirts, versus say a person that only makes $25K-$50K a year.
Also, think strategically in your marketing efforts. What geographies most frequently wear t-shirts in the summer? How about in the winter? Make sure you're locations are relevant at that time. Don't forget, while the northern hemisphere is freezing in the middle of December January, the southern hemisphere's cooking!
Lastly, when you're first starting out, focus on promoting your product. People who buy your products will help you develop your brand strategy. As your company grows, so can the message it's sending to its customers and prospects. But it happens over time, not overnight.