You will only be able to do this on a domain you own and have the ability to change the nameservers on.
I'm sure CloudFlare have a good guide on how to do it, but basically you create an account over with them if you don't have it already and then tell them what domain you want to utilize this on. Once you done so CloudFlare will let you know what nameservers you should set for your domain. On first initialization I think they copy the contents of the nameserver currently in use so if you currently have it set with ours all necessary information should be found and added to cloudflare.