Had a little experience with finding out about this, but only in the UK business area. I would copyright a design before any are sold. If you are a UK business then you actually have copyright status under intellectual property rights as soon as the design is published, i.e. posted onto the internet. And, the added bonus of being in the UK: if you post yourself your designs then this comes under a "royal" stamp (i.e. the postal service is legally bound under Her majesty) meaning that any unopened designs you send to yourself have been time stamp approved by the queen herself, and therefore you can prove time of design.
However, we then enter a more complaicated area where copyright protects the designer, and is in the form of artistic or literature works (think a drawing you have done yourself). A Trademark is a different procedure, and will apply to your company name / logo. This will protect any other indiviual from printing your name / logo onto a tshirt and selling it (think nike and swoosh tick). Then we have the area of a patent where the inventor has patented a new gizmo and wants to keep the rights to it (this costs a lot more).
copyright = design = artwork
trademark = brand = logo
patent = invented = new type fabric for a tshirt?
its a minfield of law out there, and well worth a look at, but back to the original post, if you are in the UK, its easy. Not sure about the US...
hope this helps.