There is no registration or sign-up for a Creative Commons licence. Any Creative Commons licence you want to apply to a work, you can, as long as you hold the copyright. All you need to do to licence a work under Creative Commons is choose a licence and then publicly list, display, or upload the work along with your chosen Creative Commons licence.
If you're not sure how to do that, or what it might look like, the Creative Commons Wiki has you covered:
The Creative Commons symbol is displayed under fair use.
Changing a licence once you have applied it to a work is tricky, so deciding what licence is right for your work is important.
Happily, Creative Commons is here to help: