if you are open to using a plugin then I can suggest Advanced Ads. Disclaimer: I developed it.

After you activated the plugin, just go to Advanced Ads > Settings > AdSense, type in the publisher ID (from the activation code) and enable the “Page-Level Ads” option.

The AdSense activation code should then be added automatically at the right place.

Thomas

Otherwise you can add it manually anywhere you want by inserting ONE time in header:
<script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>

Open tools>>editor and find header.php file and add before </head>

and add the rest of the adsense code anywhere you want like widget or template files:

<!-- MY-SITE-ADS RESPONSIVE -->
<ins class="adsbygoogle"
     style="display:block"
     data-ad-client="ca-pub-1111111111111111"
     data-ad-slot="111111111111111"
     data-ad-format="vertical"</ins>
<script>
(adsbygoogle = window.adsbygoogle || []).push({});
</script>

Use responsive ads to adjust to all sizes automatically.

If you want to use specific sizes with responsive ads just add this before the code:

<div class="ads_2">
<style type="text/css">
.adslot_2 { width: 120px; height: 240px; }
@media (min-width:500px) { .adslot_2 { width: 120px; height: 600px; } }
@media (min-width:800px) { .adslot_2 { width: 160px; height: 600px; } }
@media (min-width:1000px) { .adslot_2 { width: 300px; height: 1050px; } }
</style>
--- GOOGLE ADSENSE CODE ---
</div>

host: your-site-.com
username: your-hosting-username
password: your-hosting-password

Find all your website files and structure in public_html, look for wp-content/themes/your-theme-name/header.php