You start by reading the server-side development section of the IBM Worklight Getting Started page:
http://www.ibm.com/developerworks/mobile/worklight/getting-started.html#server-side-development
Specifically, these modules:
- HTTP adapter – Communicating with HTTP back-end systems
- Invoking adapter procedures from client applications
Each module has an exercise as well as a sample application.
There is more information in the IBM Worklight Information Center.