How to make the custom module effect on the node and only one content type?
If you want to add content to nodes, you can in drupal 6 use the hook_nodeapi where op is view.

As you get the node object, you can detect the node type ect. You would need to add something like this to your module

module_nodeapi(&amp;$node, $op, $a3 = NULL, $a4 = NULL) {
  switch($op) {
    case 'view':
      if ($node-&gt;type == 'node_type') {
        // $a3 indicates if teaser
        // $a4 indicates if page
        $node-&gt;content['my_additional_field'] = array(
          '#value' =&gt; 'HTML', // create html with theme or form function. 
          '#weight' =&gt; 10,
        );
      }
      break;
  }
}