Site navigation is good. Company logo is awesome. Not sure how it relates to DJ'ing, but it is very nicely done.
You should add a "t-shirts" or "Shop Now" link to your navigation bar so a person will know exactly where to go to start shopping (and know that your site is for shopping

)
Maybe make the top navigation links stand out a bit more as navigation links (maybe larger font, underlined)
Make sure the TITLE tag of every page is descriptive.
Like other's have said, the thumbnail graphics look a bit blurry.
Add a easy link to the sizing chart (or the actual sizing chart) on all the individual product pages.
Make sure when you add the add to cart buttons, that they go 'above the fold" so people can see them without scrolling.
Maybe add in your product descriptions how your shirts are printed.
I agree that the font size on the product pages should be increased.
I like the homepage layout. Maybe you could add a "newsletter" function to the site so people who are interested in your designs/company can get updates from you.
You're definitely on the right track!