My colleagues are quite right to suggest trademark protection for your logo. I write to caution, however, that a logo on a shirt does not operate as a trademark unless that logo serves a source-identifying function -- i.e. it "brands" the clothing and is not simply a neat looking design on a shirt. Absent serving as a source identifier, NO trademark rights attach to your logo. Trademark registration applications for designs on clothing are rejected as "merely ornamental" quite routinely. Which unnecessarily complicates the branding process and pushes back commercial efforts to sell the clothing.
I suggest that you read what the Trademark Office has to say about the matter at
http://tinyurl.com/cdje9p (scroll down to Section 1202.03 "Refusal on Basis of Ornamentation") and then speak with an intellectual property attorney. My initial advice still stands: for $40 or so you can register the copyright in your logo which will provide you with the right to sue anyone who reproduces it or one substantially similar to it -- even on clothes. In the meantime, you can consult with your attorney who will educate you on how to properly brand a clothing line.