Need help adding Google Adsense to individual posts

I am trying to learn how to modify my WordPress template (I am using the domain https://www.teafornewbies.com for this function).

I am trying to have my Google Adsense added at the end of each post (horizontal ad), but I can’t figure out where to type the code.

This is the code I was given:

if ( $shownads === false ) {
$shownads = true;
?>
<script type=”text/javascript”><!–
google_ad_client = “pub-xxxxxxxxxx”;
//234×60, created 12/24/07
google_ad_slot = “xxxxxxxxxx”;
google_ad_width = 234;
google_ad_height = 60;
//–></script>
<script type=”text/javascript”
src=”https://pagead2.googlesyndication.com/pagead/show_ads.js”&gt;
</script>
<?php
}

I am using the WordPress Default Theme for this.

Can someone please help me with this?

Thank you.