If the theme you’re using doesn’t come with Google Analytics integration, one of the most popular options out there is Google Analytics Dashboard Plugin for WordPress by MonsterInsights plugin here: https://wordpress.org/plugins/google-analytics-for-wordpress/

You may also add the analytics on your WordPress website without using a plugin.

Note: Before doing anything on your website, it is always a good idea to backup your site. You can use UpdarftPlus for that.

Adding Google Analytics Manually  (No Plugin)

• In your WordPress backend, go to Appearance » Editor.
• Click the header.php file on the right side below the Templates section.
• Look for the head html end tag </head> and right above that, paste the Google Analytics Script.
• Click update.

Hope that helps.

UPDATE (7/16/2020)

Checkout my video tutorial on adding Google analytics to WordPress without using a plugin.