Detect when a new WordPress post is created or published. You can do some work ONLY when a NEW post/page/custom is created using the following gist.

detect-new-wordpress-post.php

<?php 
/**
 * @param string $new_status the new status of the post eg. publish
 * @param string $old_status the old status of the post eg. draft, publish etc
 * @param WP_Post $post
 */
function prefix_do_something_when_a_new_post_published($new_status, $old_status, $post) {

	// Do something On first publish
	if ($new_status == 'publish' && $old_status != 'publish' && isset($post->post_type)) {
		switch ($post->post_type) {
			case 'post' :
				// do something here when a new post is published for the first time
				break;
			case 'page' :
				// do something here when a new page is published for the first time
				break;
			case 'your-custom-post' :
				// do something here when a new 'your-custom-post' is published for the first time
				break;
			default:
				// Silence is golden
				break;
		}
	}
}
add_action('transition_post_status','prefix_do_something_when_a_new_post_published',10,3);