You'll need to learn more about, and better define your two personas:
A) The type of company willing to pay for t-shirt based advertising
B) The type of person that will to wear it.

To do this you'll need to access communities where you think those personas would hang out (either online, or in person), and ask them questions (via an online questionnaire, or in an in person interview) that will allow you to characterize the types of people / companies that would be interested.

Once you have your two well defined personas you can better design and test your initial service around them.

For instance, maybe you've realized through your research that only bars are interested in your service, and maybe they're only interested in women wearing their advertisements. Meanwhile you might have also found that only men would be interested in wearing the advertisement shirts. In this case the idea would not be viable.

However if you find personas that do match, and there are enough of them in existance (big enough market), and/or they're interested in paying enough $ (enough to pay for the t-shirts, and your rent, and cleaning, and occasional lost shirts, etc.), then it would be viable. Not _necessarily_ the next big thing, but definitely viable.

Let me know if you'd like more detailed advice with regard to conducting interviews, designing questionnaires, how to access potential customers, or even if you'd just like to brainstorm on it some more. It's a fun idea,

best,

Lee