Here's a link to my tutorial, which walks you through :
- creating a JSON WCF Web Service from scratch (and the problems you'll want to avoid)
- adapting it to read/write SQL Server data
- getting an iOS 6 app to use the JSON servies.
- using the JSON web services with JavaScript
http://mikesknowledgebase.com/pages/Services/WebServices-Page1.htm
All source code is provided, free of charge. Enjoy.