Using a _GET gives me a debug error (over my head)

The error is occurring as the $_GET array doesn’t have the item $_GET[‘do’] in it. Therefore it throws this notice. To check if something exisits in the array try:

if( isset( $_GET['do'] ) )
   $do_that = $_GET['do'];
else
   $do_that="";

or a cleaner method would be to use short hand notation

$do_that = ( isset( $_GET['do'] ) ) ? $_GET['do'] : '';