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

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

Example of a script:

<!-- Google Tag Manager -->
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
<!-- End Google Tag Manager -->

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

<!-- Google Tag Manager (noscript) -->
<noscript><iframe src="[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