Hi Gaurav,
Just curious though .. are you planning to call a Function module from Browser like for any mobile Appl ?
If this is the case then SAP Netware Gateway will be good option for you. It provides response in oData, JSON, XML..
If you need help in how to use NW Gateway or create a Service I can surly help you out..
Please have a look to below url
http://wiki.scn.sap.com/wiki/display/EmTech/SAP+Gateway+Self-Paced+Learning
I will suggest see below post this is awesome (not related to any thing we discussed above):
JSON Adapter for ABAP Function Modules
Thanks-
Abhishek