If you want to display Google Adsense After the first post, then you can refer to the following code and see how it works. Honestly, this is a wonderful code that I have been looking for so long.

<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); $loopcounter++; ?>

// the loop stuffs

<?php if ($loopcounter <= 1) { include (TEMPLATEPATH . '/ad.php'); } ?>

<?php endwhile; ?>
<?php else : ?>
<?php endif; ?>

Thanks Web Designer Wall for this awesome code.