How to use RESTful services for migrating windows application?

I am working on a windows application in my organization. We are going to implement this application now on web as well as on mobile also. Can someone suggest what should be the correct approach for that and also it should not be too costly? We are willing to use RESTful services for this implementation. Currently application is on Microsoft framework 3.5 and sql server as backend. Can someone please elaborate more on RESTful services?