The site looks nice (I'm not sure if you changed it since your original post), but it seems to showcase the t-shirts well.
You might want to add some information about you, your company, your history, so people get an idea of who they are shopping with.
You also need to add some sort of sales/description/marketing copy to each of your t-shirts. Right now, when I click on a t-shirt design, I have no idea what it's about, why I should buy it, what type of printing is used, what type of t-shirt is used, etc.
You'll have a better chance at selling t-shirts if you describe the t-shirt, describe what makes the t-shirt special and why a person should want to have it. Put as much information as possible, since that is your only chance to "talk" to the customer about your products.
Your product images look very similar to the product shots that are auto-generated by CafePress. If this is the case, and you're not using CafePress, you may want to come up with your own unique product images.