Drupal6 form theming problem

I want to wrap my form in a table. It was easy to make in Drupal5 but I have a problem in Drupal6. My theme function don’t work.!

function mymodule_myform($form_state) {
 
  $form['description'] = array(
    '#type' => 'item',
    '#value' => t('Enter your cridentials:'),
  );  
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Submit'),
  );
  return $form;
}
 
theme_mymodule_myform($form)
{
exit('How I can get here ?');

$output.=drupal_render($form["description"]);
$output.='<table  class="my_module_form">';
$output.='<tr><td>'.drupal_render($form["first"]).'</td><td>'.drupal_render($form["middle"]).'</td><td>'.drupal_render($form["last"]).'</td></tr>';
$output.='<tr><td>'.drupal_render($form["submit"]).'</td></tr>';
$output.='</table>';
$output.=drupal_render($form);
return $output;
}