Edit the "header.php" template file! BTW,Why not do a search on DP or search it on Google? There's a same thread on DP opened 1 or 2 days ago!! He didn't mean the tagline.He meant the "meta".
A good WP blog theme should have this: <meta name="description" content="<?php bloginfo('description') ?>" /> Code (markup): FYI, in WP the tagline is the description
Your life would be a lot easier with this plugin. Just download this http://guff.szub.net/downloads/head-meta-desc.zip unzip and ftp to this folder on your site wp-content/plugins/ and activate it under plugin menu in your Wordpress admin. That's it!!!