Well...first of all you do NOT write WCF services in JSON! You (hopefully) write your services in C#. JSON is just one of many forms of serializing data to pass across the wire.
If you are asking how to write a WCF service, you could start with an article like
]. It gives a good process for writing your first service. Once your service is running and your tests for your services work, then you are ready for the next step.
Then once you have written your service you read and follow an article like this
However if you are asking someone here to write your service for you....
... excuse me because I can't stop laughing.