Google Tag Manager provides a convenient method to control the code snippets you would like to include within your store - without needing to modify a theme's code every time. Please be sure to watch the Google Tag Manager Getting Started video to get up to speed on how to get going with the tool.
Upon signing up for your Tag Manager account, will be presented with a code snippet that looks like the code snippet in the following image. The next step will be to add this code into your theme as instructed:
Adding the Google Tag Manager code to your shop
Adding a code snippet to your theme is broken down in the following article. Please read the article and follow the steps: How do I add a code snippet to my shop?
When inserting the code into the 'theme' template, be sure to paste the snippet just after the opening <body> tag, as demonstrated in the following screenshot:
Note that depending on your theme, the exact code will look different, but it's the opening <body> tag you are looking for. From here, save changes and you're done!