I'm sorry but it's not going to work.
People would rather get NIKE than a smaller unknown brand of shirt.
I've personally never bought, nor even though about buying a Nike shirt before.
And honestly stating, I've maybe seen one Nike shirt in person in my life and that was when I used to play basketball.
I used to advertise and model for a small clothing company of who's designs I preferred over the shirts I had previously bought.
If you want to grow at a seemingly fast rate (for this type of business anyways), make some "prototype" designs, get a few of them printed on some t-shirts, and work your way onto social networking sites.
Find some people with a very high friend count (such as those scene kids on MySpace) and make a deal that you will give them a free shirt if they take a picture with them wearing it and provide your company's web address underneath the picture.
One company that I worked with before had used that method and jumped from a little over 500 friends to over 70,000 in a matter of two months (on MySpace, of course)
It's just a suggestion as to what you could do to spread the word, assuming that your designs fit the wants of an every-day teenager.