How to create RESTful Web services provide a simple JSON interface

I am building cross paltfom xamrin forms app which scanning barcodes using  ZXing.Net.Mobile.Forms.

Now, I need to store scanning data in Json file if there is no wi-fi or Internet on devices.

• for this How do i create RESTful web services to detect, read, and generate 1D and 2D barcode in images by using JSON interface for Xamarin app