Need a simple alternative Api Authentication system in an already built Api using Asp.Net MVC4 C#

Question

I have completed working on an Api using C# in a Asp.Net MVC4 application project. Now there is a need to add an authentication process in the completed Api. I need guidance to make the authentication using username and password and then use httpheader, httpwebrequest or httpclient for adding header authorization system. I am not that experienced in this, so I am thinking of any easy/simplest way to add the authentication system to the Api via username and password. Thanks.