You don't, unless your site scales beyond a point where you start to experience performance issues because it is overloaded.
The reason for this is simple: Having resources on a CDN requires the client's browser to start an additional session, complete with DNS lookup and everything, to download them. This usually costs performance.
Beyond that, a content delivery network basically is just a fancy name for a second (third, fourth...) server: Load your main site from http://example.com and reference the static content from http://cdn-example.com. On a large scale, there is obviously more to it, but that's the fundamental.
Something else: Having to load a lot of resource files is usually a sign of weak site design: Try to combine your resources into as few files as possible. It should usually be possible to condense all your JS and CSS in only a few files.