Major elements imo are:
1) Simplicity -- The user should be able to figure out what you are selling within the first 5 seconds that they visit your site. Avoid flashy animations and advertisements unless you know how to effectively use them.
2) Navigation -- Your navigation needs to be user friendly and efficient. Also, an organized site map, with a hierarchy of where your links go (parent-child linkage for seo)
3) Consistency -- Throughout your pages, you should be using same layout techniques and design....which is great and easy if you use CSS.
4) Security info -- You need to make sure that you have a professional layout and reassure the customer that the site is secure. Having a crappy design and no contact info will make a lot of customers think that you are a two bit operation....and if you are a two bit operation, then play the part of a big timer.
5) Like solmu said, Make sure you customers know that you are selling t-shirts the moment they get to your site. Don't clutter it with text or other unnecessary garbage. Pictures speak a thousand words, visually speak to your customer instead of making them read a lot.
6) Backend structure -- Make sure you site is compatible across platforms and check all resolutions. Also, you need to be aware of your doctype and meta tags for seo as well. I don't know much about seo, but it should be one of your top priorities imo. A good website is one that looks good on the front end, but also has a smart backend.
Thats my 2 cents. Hope it helps