For announcements you can use the hook I posted before. For other pages (index, about, clientarea, register, pwreset, supporttickets...) it's easier. Begin by creating meta descriptions in your language file like follows (use language overrides!):
$_LANG['metadescription']['index'] = "We are the best hosting company";
$_LANG['metadescription']['clientarea'] = "Manage renewals, download invoices, submit tickets...";
$_LANG['metadescription']['register'] = "Register an account with us";
Use file names (index.php, clientarea.php, register.php...) as keys. Now open your templates/{YOUR_TEMPLATE}/includes/header.tpl and add the meta tag in your <head> tag.
{if $LANG.metadescription[$filename]}
<meta name="description" content="{$LANG.metadescription[$filename]}" />
{/if}
Meta description automatically shows up if there's an available description for current page.