How do I implement Authentication In ASP.NET MVC and Web API?

I want to implement authentication and authorization in my MVC and Web API projects.

For cross browser and mobile access, we are implementing Web API into our project. In our Views, we are using normal controllers. For data we are using API controllers. How can I achieve role-based access and authorization in ASP.NET MVC 5?

I have added the authentication tag to the web.config. When I run the application, I'm getting the following error:

HTTP Error 500.19 - Internal Server Error The requested page cannot be accessed because the related configuration data for the page is invalid. Error Cod: 0x800700b7