help me radomize the post

<?php
	$counter = 0; $counter2 = 0;
	while (have_posts()) : the_post();
	?>
<?php $counter++; $counter2++; ?>
        <div class="grid_6 <?php if ($counter == 1) { echo 'alpha'; } else { echo 'omega'; $counter = 0; } ?>">

hello guys, can u pls help me
the code above is in my index.php and i want to display random posts
can someone help me ?