in web.config set the login page url
<authentication mode="Forms">
<forms loginurl="~/User/Login.aspx" timeout="2880" />
</authentication>
in login page code, after successfully login you can redirect using
Request["ReturnUrl"]
string strRedirect = Request["ReturnUrl"];
if (strRedirect == null)
strRedirect = @"~/Default.aspx";
Response.Redirect(strRedirect);