Add a specific category to a query

Hi,

I need to only display a certain category from a query:

$letters = $wpdb->get_col(
“SELECT DISTINCT LEFT(post_title,1) AS first_letter FROM
$wpdb->posts
WHERE post_type = ‘$post_type’ AND post_status = ‘publish’
ORDER BY first_letter ASC”
);

Please could someone tell me how to do this? I haven’t got the first clue!

Thanks.