How to select custom data from WordPress DB to android SQLite

I am developing an Android application which is needed to retrieve custom data from WordPress DB and save them to SQLite DB. From my own Web App, I am receiving posts in 3 below steps: 1- The Mobile application sends request containing the required post(s) ID to the web app. 2- Web app, executes query where id is equal to requested ID. 3- Web app parses selected data to JSON format and response the JSON file back to Mobile Application.

Now, I need to receive such a data from WordPress DB, parse it to JSON file and then send the file back to mobile. Please help me to solve this problem. I am really in need of it.

Thanks.