How do i get post thumbnail using attachment code

I can’t tell if you want a link to the thumbnail or you want the image to be the thumbnail.

To get the image assigned to the post as the thumbnail image, use get_the_post_thumbnail:

  $thumb = get_the_post_thumbnail($post_id, 'thumbnail');

To get the thumbnail using the attachment ID use wp_get_attachment_image:

$thumb = wp_get_attachment_image( $attachment_id, 'thumbnail' )

Not that that will return an empty string if the requested image does not exist.

To get a link to the image using the attachment ID use wp_get_attachment_thumb_url

$thumb = wp_get_attachment_thumb_url( $attachment_id ); 

Or wp_get_attachment_image_src:

$thumb = wp_get_attachment_image_src( $attachment_id, 'thumbnail' ); 
if (!empty($thumb['url'])) {
  $thumb_url = $thumb['url'];
}

Based on a comment below: How to “get the thumbnail of the post and make it link to the first attatchment under that post”

$thumb = get_the_post_thumbnail($post->ID, 'thumbnail');
if (!empty($thumb)) {
  $args = array(
    'post_type' => 'attachment',
    'numberposts' => 1,
    'orderby' => 'menu_order',
    'order' => 'asc',
    'post_status' => 'inherit',
    'post_parent' => $post->ID,
  );
  $attachments = get_posts($args);
  if (!empty($attachments[0]->ID)) {
    if(wp_attachment_is_image( $attachments[0]->ID )) {
      $title="<a href="".get_attachment_link($attachments[0]->ID).'">'.$thumb.'</a>';
    } 
  }
}
if (!empty($title)) {
  echo $title;
} else {
  the_title();
}