How to select a specific page

From WordPress Codex Get Posts.

<?php
$the_slug = 'my_slug';
$args = array(
  'name'        => $the_slug,
  'post_type'   => 'post',
  'post_status' => 'publish',
  'numberposts' => 1
);
$my_posts = get_posts($args);
if( $my_posts ) :
  echo 'ID on the first post found ' . $my_posts[0]->ID;
endif;
?>

That way you can get a post or page by its slug.