If you're talking about the built-in Logout functionality when you're logged in to edit/admin mode, you could rewrite the logout route handler and append a redirect to your login page. 

Alf has an example of it here: http://talk.alfnilsson.se/2014/01/29/hooking-up-to-the-episerver-quicknavigator/

If you're talking about a custom Logout button that you have in your templates somewhere, you could just add a redirect in your controller when that button is clicked. 

No i was talking default episerver logout. It has navigate to my custom login page after logging out.

Unless you want to have one login page for logout, and one login page for first-time login, you can change the loginUrl in web.config
Or do you want to have 2 login pages?

<add url="~/util/logout.aspx" mappedUrl="~/Login?ReturnUrl=/" />

I was using this to map url 

and in controller 

public ActionResult Login()
{
FormsAuthentication.SignOut();
return View(string.Format("~/Views/StartPage/Login.cshtml"));
}

i am doing this in PageControllerBase class but it doesnt load the css at all in the page

Personally I'd use the IIS url rewrite module to redirect "/Util/logout.aspx" to your login page.