echo"Our newest member is <a href='$news/'>$new</a><br />';
It's very simple, the variable $new becomes the username from the SQL Query :)
And as far as using the same variable for each line, it will make no difference, but it is bad practice, as any data in the variable will be overwritten, especially in this case for pulling SQL Results
Not always, the default file that is loaded is called the "DirectoryIndex". Your server (I'm guessing Apache) will use what are called .htaccessfiles.
When apache receives the request for a page it looks for the closest .htaccess file (often they can be cached in RAM) and uses those guidelines to display the correct page and headers.
If you want to change what default page is loaded just put this in a file called ".htaccess" in the directory you want to change. Note: You will not be able to just rename a file to ".htaccess" on Windows, Open->File->Save As.
# Note, if you want to have it check for multiple files just
# list them with a space between.
DirectoryIndex index.php home.php install.php error.php