This example will look for the first image in a post, and, if there is one, will display a 75 x 75 pixel thumbnail of it. You can pass an array with width and height, or a size keyword (see here: http://codex.wordpress.org/Function_Reference/wp_get_attachment_image).

gistfile1.php

<?php

$images = get_children('post_parent='.$post->ID.'&post_type=attachment&post_mime_type=image&numberposts=1&order=ASC');
if(!empty($images)){
  echo wp_get_attachment_image(reset($images)->ID, array(75,75));
}

?>