Very similar to my previous sites - I didn't have anything in there to really make people who landed there to want to click on a product & explore.
I'm not the expert but from my experience I had over 1000 visitors finding my site & the same conversion problem you're having. Once I started brightening it up with banners / pre-decorated products etc. (it's an ongoing project!) my conversion rates increased.
Maybe worth considering.
I like the site design I think its really clean and professional looking.
However you should probably make those banners into links so it takes you straight to your products page and also move the Create Custom Tee Button up higher but just my .2c