Depends on the thing. Chances are, whatever it was is not unique enough to warrant a patent. A trademark has to do with a mark to do with trade of goods / services, like a unique business name or image, so probably also not applicable, unless you want to trademark your logo or something. You would register it with the Patent and Trademark office. So a copyright is what you're looking for, in which case most intellectual property is considered copyrighted at the time of creation, there's nothing special you need to do. You can register your copyright with the U.S. copyright office, but it probably isn't worth the time and effort, unless the thing you created was highly valuable and highly desirable.

Thank for the answer. If I register it with the US copyright office will that copyright hold worldwide?

I sent them an email asking for more info.

Very disappointed to learn that I cannot copyright an online calculator that I have spend months perfecting. One of my more established competitors can simply take this whenever they want and I lose the competitive advantage.

Devices and similar articles designed for computing and measuring cannot be
copyrighted. Common examples of uncopyrightable computing and measuring
works include slide rulers, wheel dials, and perpetual calendar designs.
Ideas, methods, systems, mathematical principles, formulas, and equations
are not copyrightable, and the same is true of devices based on them. Printed
material on a device--for example, lines, numbers, symbols, and calibrations,
as well as their arrangement--is likewise not copyrightable, because such material
is necessarily dictated by an uncopyrightable idea, principle, formula, or
standard of measurement.
To be copyrightable, a work must contain a certain minimum amount of
creative authorship in the form of original literary, artistic, or musical
expression. A computing or measuring device as such ordinarily contains no
copyrightable expression. It does not communicate facts or ideas but, instead, is
a means for arriving at an almost unlimited number of readings or results. To
the extent that the contents of a device are predetermined by its function, they
lack creative authorship.

http://copyright.gov/circs/circ33.pdf

That's too bad. But thanks for sharing what you learned.