Before you choose a license for your work, you'll want to carefully consider your options. CC license are non-revocable, so it's very important to take this step seriously. Once you have an idea of how you'd like others to use your work, you can attach the license of your choice.
Once you've selected your license, CC provides you with code you can embed in your website (which helps with search engine discovery) and icons that you can insert into the item itself. If you are marking a document meant to be shared offline, use the "Help Others Attribute You" box on the license chooser to get a formatted license statement. If you are uploading content to a third party website (like Flickr or Texas ScholarWorks), check to see if there is an option in the upload process for designating a CC license.
The Creative Commons wiki has a helpful page about marking your work with a CC license.
Example of a license on a website
Example of a license on a text document
