What type of work can be copywritten and can I use one copyright for all of my productions?

As soon as a qualifying "work of authorship" is created and fixed onto some medium copyright attaches to the work -- you don't have to do anything to "copyright" a work other than complete the work. Read the Copyright Basics handout that Ms. Koslyn cites.

It is often -- but not always -- worthwhile to register with the Copyright Office the copyright that attaches to your works. Registration is worthwhile if you intend to sell copies of a work or if the work somehow provides you with a competitive advantage. Otherwise, registration may not be necessary (though it's always best to ask a intellectual property attorney to help you make that decision).

As Ms. Koslyn notes, the copyright in "designs" must be registered separately (unlike newspapers, other periodicals, photographs, and a few other types of works). Note that a "design" is, in copyright lingo, either a "pictorial" or "graphic" work (the distinction between the two is not important). Registration is fairly simple. You should, however, have a copyright attorney prepare at least the first registration form -- which you can then use as a template to register similar works of that type. Good luck.

See the Copyright Office's circular linked below for what types of works can and cannot be copyrighted. It also describes certain works that can be copyrighted as a group, such as photographs and periodicals (see for example, http://www.copyright.gov/circs/circ62b.pdf).

Each of your t-shirt designs must be copyrighted individually.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.