At this point I assume you have created your t-shirt designs and uploaded them to your company of choice so now lets go over some ways to promote and market your t-shirts.
1. Wear your own shirts!
What better way to promote your designs is to wear your own shirts and carry business cards to hand out to those who compliment you on your great design.
2. Post your t-shirt store on directories
Try posting your t-shirt web site on T-Shirt Directory.
3. Post your designs on design directories or topsites.
Try posting a design at T-Shirt Expo Design Directory.
4. Exchange links with web sites who might be interested in exchanging traffic.
Lots of sites would be happy to trade links to help generate traffic especially if you have great looking t-shirts.
5. Use Pay Per Click (PPC) advertising
Paying for traffic is a great way to get customers. The two most popular PPC sites are Google Adwords and Overture (Yahoo Search)