http://www.keytlaw.com/Copyrights/benefits.htm said:United States copyright law does not require that the creator of a work register the work with the U.S. Copyright Office. U.S. law provides that when a work is created, the work is protected automatically by U.S. copyright law without the need to register the work or display a copyright notice. Copyright protection attaches to a work when it is fixed in a copy or phonorecord for the first time, whether or not the author uses the "©" symbol or the copyright notice.
Do you want to know what the best way to prove it's yours is? Seal it securely in an envelope and post it to yourself. If you can, get a premium postal service too (Royal Mail [in the UK] seem to be getting a bit tardy...) like Next Day Recorded (signed for) delivery. Sure, it'll cost a bit more, but worth it if something happens.
Obviously, for digital content, I don't think that digital signatures are recognised legally for that sort of thing yet, but you could always print it off etc.
No, there's a way to digitally "sign" files. You can't change it after the signature is added otherwise its void-ed. Along with the signature, the date and time are recorded so it's possible to trace it. The only thing is that if you were to change the date of the system signing the file then you could effectively back-date the signature and fake any.
As for creative commons, that's not exactly copyright although it's similar. You can publish work under the CC license but other people are allowed to use it under the terms you set (eg, attribution to you, no derivatives etc).