This is not an in-depth analysis but a couple things struck me immediately. These are simply my opinions, others may not agree.
1. Rather than have the product categories listed on your main page why not also put some shirts from various categories on the main page to engage the customer immediately. This would also liven up your home page a bit.
2. The product pictures. Please don't be offended but it's obvious you took those pictures with an older digital camera. The quality is not there and the majority of the shirts are very hard to read. I would recommend either using a shirt template and photoshopping your designs onto it or hiring a professional photographer and model for your shirts.
3. The prices. Eight dollars seems awfully low. I'm sure your paying customers don't mind but this might turn some people off; they see the low-quality shirt picture and then see the $8 price tag and make the assumption that it is a low-quality product. Rodney mentioned once in a thread that actually raising the prices of shirts had increased his sales. I bet if you raised the price to at least $14.99 and fixed the issue addressed in #2 that you would see a rise in sales.
I hope this helps. I think it's great that your sight is up and running and you are selling. Don't take anything I said personally, you are farther along than me. Keep it up!
- Phil