session creation for login and logout

At login, when user is authenticated:

Session["IsLoggedIn"] = "Yes";
//OR
Session["LoggedInUserId"] = "User621";

At the time of logout, clear the session:

Session.Clear();
//OR 
Session["IsLoggedIn"] = null;

If you want to check if user is logged in or not, just see if session value exists. If it's null then not.