Entity framework - WCF - return JSON how to do this?

Entity framework - WCF - return JSON how to do this?

I have all the POCO entities produced from my database. I created an IXXX
interface, a XXX class to define the structure of the table I want to
return from my service, and a XXX class to do the query and the return
part for the interface.
My question is regarding the elements I need to add to this setup in order
to return clean JSON from my web service.
I'm a beginner so all points of view are welcome. Thanks!