Step 1: Firstly login to https://www.google.com/recaptcha/ and generate your website Unique reCAPTCHA code.

Step 2: Go to your WordPress admin panel. Open up Edit templates. Find and edit the header file. Paste this snippet before the closing </head> tag on your HTML template.
<script src='https://www.google.com/recaptcha/api.js'></script> and save.

Step 3: Open up and edit your contact-us.php file. Paste this snippet at the end of the <form> where you want the reCAPTCHA widget to appear.

<div class="g-recaptcha" data-sitekey="your_data_key_that_you_generated_in_google"></div>

That's it, now the Google reCAPTCHA shows up on your contact form.

take help of this video tutorial - https://www.youtube.com/watch?v=XjN0j4JQqVI

Hope it was helpful! Good luck!