Use FormsAuthentication and set the LoginUrl to your Login.aspx page. Something like:
<authentication mode="Forms">
<forms loginUrl="~/Login.aspx" protection="All" timeout="600" slidingExpiration="true" domain=""/>
</authentication>
Then after they login set the authentication cookies using FormsAuthentication.RedirectFromLoginPage().
This way any request to your site if they have not been authenticated they will be sent automatically to Login.aspx.