Step 1- Set the form authentication mode equals to form

Step 2-In web.config add following code

Inside system.web

    <authenticationmode="Forms">
      <formsloginUrl="~/Home/SignIn"timeout="2880"/>
    </authentication>

Step 3-In this step we need to controller for checking authentication. Here we check that user is authorize or not.

In home controller add following code

        [Authorize]
        publicActionResult About()
        {
            return View();//This view is viewed by authenticated user only
        }
        [HttpPost]
        publicActionResult SignIn(Login Model)
        { 
            if ((Request.Form["txtUserName"] == "Manish") &&
         (Request.Form["txtPassword"] == "Mindstick"))
            {
                FormsAuthentication.SetAuthCookie("Manish", true);
                return View("About");
            }
            else
            {
                return View("Index");
            }

Action decorated with Authorize cannot be called by unauthorized user.