Step by Step guide on how to set up google re-captcha:
Google re-captcha is one of the greatest online tools developed to fight anti-spam on any website. Google re-captcha is easy to install and configure. It can easily block the spammed traffic with its artificial intelligence. Google re-captcha can easily identify and differentiate between the human and bot traffic.
Follow the steps mentioned below to set up Google re-captcha on your WordPress website.
Step 1:
Open your computer browser and type https://www.google.com/recaptcha/intro/ in the address bar and click on “Get reCaptcha” button.

Step 2:
Post clicking on “Get ReCaptcha” you will need to login through your Gmail Id and Password. Post that you will come across a new site registration form where you can choose the type of Google re-captcha you wish to use. Mostly people opt for reCaptcha V2 Option. you will also require to enter all your domains that you wish to set up Google re-captcha on. check the image below.

Step 3:
After filling out the details mentioned above you will have to accept the terms and conditions of Google reCaptcha policy and proceed.
Later you will get the site key & secret key as shown in the image below.

Step 4:
In order to do the client side integration you will have to put the first snippet before the closing </head> tag on your HTML template.
The second snippet needs to be placed in the contact form or any other form where you want the reCaptcha widget to appear.

Also Learn how to block spam on WordPress site
Step 5 :

Now you will be required to configure the server side integration. In order to know how to do the server side configuration follow the detailed description on coding part here
You can check detailed guide on Google ReCaptcha provided by Google here