Site architecture (ASP.Net vs. PHP)

OK, I'm coming from a PHP background and am haveing a hard time figuring out parts of how to architect an ASP.net site.
I am trying to tie the root of the site I'm working on to store the username of a visitor in a database and if the site cannot get the username from the browser than it should go through a manual log it an authentication (this is being used for an authentication portal for multiple intranet applications).

Here is the PHP psudeocode for what I'm trying to do:
 
1 if($_GET['app'] == '')
2 {
3 raise_error('No App');
4 }
5 else
6 {
7 // handle no passthorugh authentication (firefox, opera, etc.)
8 if($_SERVER['username'] == '')
9 {
10 manual_l ...