I have come to believe that anything that you wear directly or indirectly sends a message the to people who look at you and you interact with. Many call it fashion sense, this sends an indirect message on the brands you prefer (some might even call it ‘taste’).
However, this doesn’t convey what you believe in, or what you want to share to everyone else. Make them aware of what messages, ethos, principles, values you want to share. The most direct way would be to design and print/manufacture them yourself. Since, I’m not great at design nor have enough experience in making t-shirts. I chose to share messages of organizations / companies that I love. As an added bonus I get to support the companies I love by buying their official merchandise.
This is a non-exhaustive list of merchandise, that I own / want to buy from. I want to make a promise to only buy clothes this way.