Welcome to the wonderful world of SEO. Here is my tip for the day (and year)...
As you said, you are in a competitive market space selling t-shirts, so use your competition to your advantage. How you might ask? Look at who your competition is using to obtain backlinks and try to leverage the same sites whenever possible
Go to Google and do searches on the keywords you want to target. Once that is done, take the websites on the first couple of pages that you consider to be your direct competition. From there, go to a site that checks backlinks. (I typically use [snipped url's]).
Take your list of competitors and run them through one of these backlink checkers. Now you have a huge list of sites that your competition is using for backlinks. TIP: Take each list of backlinks and put it into an electronic spreadsheet.
Now that you have this massive list of websites your competition uses for backlinks, start checking which sites will be easy to create free one-way links to your sites, e.g., business directories, obscure social networking websites, discussion forums.
What you typically find out is that each competitor has a few low-hanging fruit websites to obtain backlinks from; so if you get a unique list of 5-10 backlinks from each competitor that you too can leverage, you may be able to build up quite a good list for yourself when you combine 5-10 websites per each competitor.
I've used this technique time-and-time again and I'm amazed that almost everyday I find new websites in which I can obtain one-way backlinks from.
Note: this takes time in both research and execution, so patience is needed.
Also, with the two websites I've provided to check backlinks, the first one shows whether the backlink has a "no follow" tag. Stay away from these types of links since they have little to no SEO value.
[edited by: brotherhood_of_LAN at 2:22 pm (utc) on Mar 31, 2010]
[edit reason] No URL specifics are per the ToS... thanks [/edit]