Best way to start writing an api for SQL server 2017

I am new to writing API's, and was wondering where the best place to start was with writing a RESTful API for a locally hosted SQL2017 server. I have written an app using angular, and wish to access my data that is located on the local network (installed on a windows 10 machine) through a API.
I already have a VB.net project that connects to the database (each practice has a local server and program that connects through the local network).  I am writing a app to give more functionality to the clients, and need to access this data from the newly written app (using ionics and angular, writen in visual studio code).   As I am new to API's, I was wondering where the best place was to start, or any good examples on how to build a basic api.  I would also like to know if I can integrate this into the existing VB.net project, as the project and sql server are already at clients.