Know-How Exchange : Copywriting

Go here to do it. https://www.copyright.gov/

In most countries copyright is automatically applied - there is no need to do anything special, you have copyright just from having created something.

However, for legal purposes you may need to show the date of creation - which can be more problematic with software. You may need to periodically put the software on a physical media - paper or CD which can then be mailed to yourself to establish a creation date.

Note though that copyright for parts of software (as opposed to the whole package) is difficult to enforce as SCO's attempt to claim Linux contains copyright Unix code is showing, and as did the Look and feel cases between Microsoft and Apple. In the US patent protection is considered stronger.

Copyright gives you rights over the reproduction of copies of your work or parts of the work - like literature or design. You have to show some thing else doing the same as you is a copy and can be seen to be a copy, and not simply a re-invention or independent development (patents protects against re-inventions). Copyright doesn't protect function as such.

Having established a date of creation, that should be universally applicable to demonstrate authorship. Note though that some (non-Western) countries have a reputation for turning a blind-eye to fakes and copies.

Saul