Google Tag Manager

To install Google Tag Manager on your website take the following steps within the Google Tag Manager dashboard:

Go to Admin > Install Google Tag Manager
Copy and paste the code as high in the head tag of the page as possible

Example of a script:

<!-- Google Tag Manager -->
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','[TAG_ID]');</script>
<!-- End Google Tag Manager -->

Additionally, paste this code immediately after the body opening tag:

<!-- Google Tag Manager (noscript) -->
<noscript><iframe src="https://www.googletagmanager.com/ns.html?id=[TAG_ID]"
height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
<!-- End Google Tag Manager (noscript) -->

Instead of TAG_ID use your own tag id.

You can find more information here