Creating Rest Web Service with an existing asp.net mvc 5 app

Question

I have an existing asp.net mvc5 web application that uses the standard UserManager classes to perform authentication using the Entity Framework, so i need now to create a RESTful web service to do authentication and to store and update user details.

I'd like also to continue using the ASP.NET identity AspNetRoles table to manage the roles for user once he/she logged in.

Any idea please or do you have any links that can help me cause i'm new in the web service thing.