When you create something, like a book, script, song or poem, you have an automatic copyright. Proving that you are the copyright holder, however, is important if a dispute over the original creator of the work arises. Proving you are the owner of the work is simple and costs very little; in fact, the solution is as close as your nearest post office.
Once your creative work -- song, book, poem, for example -- is finished, make a copy of it. Printing a hard copy of a book or script or printing out song lyrics and making a recording of the music is vital. Make a full copy of the entire work.
Package the creative work in a safe shipping container or envelope. It should be something that is sturdy and can last many years. You can get a shipping box for free from the United States Post Office.
Ship the well-sealed box to yourself. The postmark date on the container proves that you were the creator of the work and when you created it. In the future, this can be used in court to prove you have the copyright. The only cost of doing so is mailing the package to yourself.
To go an extra step, you can register the work with the United States government copyright office. There are fees associated with filing and the process is more complex and time consuming, but you'll have the peace of mind in knowing your work is protected. If you are an author being published or musician selling albums on a large scale, this is a good idea.
Include all parts of your original work. Only what is included in the package is proof of copyright in a court case.
Never open the package. Once it is open, it is no longer usable in a legal dispute.