Just open the comments.php file from your theme. Find the following line (or similar)
<?php foreach ($comments as $comment) : ?>
Add this lines just above it
if (isset($_GET['comOrder']) && $_GET['comOrder'] == 'Newest' )
$comments = array_reverse($comments, true);
so your code looks like this:
<?php if (isset($_GET['comOrder']) && $_GET['comOrder'] == 'Newest' )
$comments = array_reverse($comments, true);
foreach ($comments as $comment) : ?>
and then make sure your newest first
like points to your current location with ?comOrder=Newest
UPDATE:
In case you are using wp_list_comments
then simply add reverse_top_level
parameter set to true when newest first link is clicked so something like this should work:
$args = array('type' =>'comments','callback' => 'mytheme_comment');
if (isset($_GET['comOrder']) && $_GET['comOrder'] == 'Newest' )
$args['reverse_top_level'] = true;
wp_list_comments($args);