Google ReCaptcha is a world popular captcha plugin. It is used on all across the web, and is not just limited for use in Joomla 3.8. It is helpful to implement spam protection on your Joomla websites. First we have to get public and private keys from google recaptcha site and generate it. This is a free service, but you will need a Google account to create captcha keys for your main domain.
Configuring ReCaptcha for Joomla 3.8:
Step 1: Log into your Joomla 3.8 Site Administrator.
Step 2: Now go to your site Global Configuration settings and set Captcha - ReCaptcha as your default captcha plugin(You can find it from Global Configuration settings under the Site tab)
Step 3: Then go and click on Extenstions -> Plugin Manager from top menu of Joomla.
Step 4: Using the Joomla filter feature at the top of the page and here type the word recaptcha. Then click on the result 'Captcha - ReCaptcha'
Step 5: On the Details tab, makesure that Status is set to Enabled.
Step 6: Find Public Key and Private Key under Basic Options tab and add your site domain keys where registered from google captcha admin. You can get Google Public Key and Private Key - Click here.
Step 7: Now you can choose default Google captcha themes and after setting all options, click the Save button in the top left.
Step 8: Finally, Test ReCaptcha to ensure it is working! You can do this by visiting your site Joomla User Registration page. Look at the screenshot.