Use UIComponent.getClientId to get the actual Id of the component you're interested in.
Render this ID on your page inside a javascript fragment (js function call, variable, whatever) so you can pick it up with jQuery.
There are various ways to use EL on a Facelet to achieve this effect, but it largely depends on what you want: eg a single component's onclick handler, or a random component's Id in a script that executes at the bottom of the page...
