I have a problem with wpdb

Hello i have a problem with wpdb

Warning: Missing argument 2 for wpdb::prepare(), called in /home/moviemro/public_html/wp-content/themes/Moviem/tagging.php on line 33 and defined in /home/moviemro/public_html/wp-includes/wp-db.php on line 990

My code is :

$first_char = $_GET["litera"];

$postids=$wpdb->get_col($wpdb->prepare("

SELECT      ID

FROM        $wpdb->posts

WHERE       SUBSTR($wpdb->posts.post_title,1,1) = %s

ORDER BY    $wpdb->posts.post_title",$first_char));

}

I don’t know how make it,please help me