Creating Database connection API for mobile apps

Question

I have a running database for web application built using C# and mvc 4, and we want to develop mobile apps. Now I have to create a secure api/web service that connects my mobile apps with my existing database and transfer data between them without need of creating a new database.

Even if i create a web service (completely no knowledge of implementation), How will I be able to get request from mobile and send the response back with result or error message? 

I am totally new in this concept and require a guidance in the right direction about how to implement same and what necessary things should I consider for this. Please suggest some good links that deals with Implementation for C# MVC. 

Thanks