forms authentication.

hii
I am new to asp.net.plz help me out.
 
 I have created login page login.aspx .In web.config i gave login url etc..I have given roles in authorization.I have one more page called default.aspx.What I want is If I set default.aspx as a start page...I should go to loginpage.
for this I have set default.aspx as start page,and in default.aspx i wrote

          if (Request.QueryString["ReturnUrl"] == null)
          {
              response.redirect("login.aspx");
          }
it works and now i am gng to login page. but after going to login  page after entering user name and passwording on presing login button I a ...