Creative Commons is a licensing scheme which allows authors to license their work so that others may re-use it without having to contact them for permission. You may not add a Creative Commons (CC) licence to a work for which you do not hold the copyright. There are several different CC licences, all of which require attribution of the author (denoted by BY in all licences' names). For further details see the Creative Commons website.
Note that if you have assigned copyright to the publisher, you will not be able to add a licence to your work, including any earlier versions of it, unless you ask for permission from the publisher.
If your work contains third party copyright material, you must ensure that you have the copyright holder's permission to make their work available under a Creative Commons licence unless an exception to copyright (such as quotations for the purpose of criticism or review) applies. If you have questions about this, or would like assistance, contact Library Staff.