Adding a plugin to a hook

I’m learning how I can make a plugin. And at the moment I have this, mainly from the documentation on www.ads-software.com:

function fp_get_latest(){
	$query_latest = "
    SELECT wposts.*
    FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
    WHERE wposts.ID = wpostmeta.post_id
    AND wpostmeta.meta_key = 'portfolio'
    AND wpostmeta.meta_value = '1'
    AND wposts.post_status = 'publish'
    AND wposts.post_type = 'post'
    ORDER BY wposts.post_date DESC
 ";
$fp_latest = $wpdb->get_results($query_latest, OBJECT);
return $fp_latest;
}
?>
add_action('get_header', 'fp_get_latest');

But when I activate the plugin and visit my blog, I get a HTTP-error 500.
What do I wrong?