Give people information, or even be more creative. Get people to share pictures of themselves wearing the t-shirt. Create a community of people who are interested in your product. Get a famous person to wear it and share that picture online. Do something different, do something unique. Show Google that you are different and better than the other search results.
Your SEO keywords are the keywords and phrases in your web content that make it possible for people to find your site via search engines. A website that is well optimized for search engines "speaks the same language" as its potential visitor base with keywords for SEO that help connect searchers to your site. Keywords are one of the main elements of SEO.