Edit web.config:
The web.config entities for REST is quite different from those for SOAP, but very similar to those for Ajax-enabled WCF Service, the only difference is, Ajax-enabled WCF has "enableWebScript":
Add WebGet attribute to all methods in contract interface, like:
[WebGet(UriTemplate = "Hello", ResponseFormat = WebMessageFormat.Json)]
Add this attribute for REST implementation class:
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
Test in Fiddler:
By now, it is a functioning WCF RESTful service, and response json format data. You can test it in Fiddler.
Great References:
http://www.ajaxlines.com/ajax/stuff/article/return_json_from_ajaxenabled_wcf_service.php
http://www.robbagby.com/rest/rest-in-wcf-blog-series-index/
