Add JavaScript to your theme

Can also check if node type is correct before inserting a JavaScript.

Snippet:

<?php
#in template.php:
drupal_add_js(drupal_get_path('theme', 'your_theme_name').'/js/main.js');

#Or if node type is being viewed:
if ((arg(0) == 'node') && is_numeric(arg(1))) {
    $node = node_load(array('nid'=>arg(1)));
    if ($node->type=='forum'){
        drupal_add_js(drupal_get_path('theme', 'your_theme_name').'/js/forum.js');
    }
}
?>