To your first question, I'm not a lawyer, but I don't see why copyrighting t-shirt designs would be any different than copyrighting any other type of intellectual property.

To your second question, there's no way to answer that with any kind of authority. There are too many unknown factors -- including who the other person is, and how much money they have, and how easily they are intimidated, and how good your attorney is, and how good his attorney is, and what country they are in, and how strong your case is, and... see what I mean?

I too am not a lawyer.

What I have been told by my favorite lawyer is that in order to collect punitive damages your T-shirt, or drawing, or photograph, or written piece (any intellectual property) must have 3 things on each and every article that goes out...

1. © or the word copyright
2. the name of the copyright holder
3. the year

Without all three, you can only collect the dollar amounts for any sales the infringer makes.

:-Don

All about copyright:

http://www.copyright.gov/help/faq/

All about trademarks:

http://www.uspto.gov/trademarks/basics/definitions.jsp

Short answer: No, you can't copyright your tshirt designs.

You may be able to claim the marks under trademark protection.

If you don't understand the basics of intellectual property you are not likely in a position to adequately defend your rights legally. Not a judgment, just the way the world works.

Hope this helps,

Brian