First, you will want to enable the Are You Human captcha option on your site. If you can't find the page for captcha options, search "captcha" in your concrete5 Dashboard's Intelligent Search. You can select Are You A Human from the dropdown menu labeled "Active Captcha".
This is the dialog you will see after selecting the "Are You A Human" option.
The next step is to add your captcha keys. First, run through setting up your account on Are You A Human. Be sure to enter the correct URL for your site in "Site URL", or the captcha won't work.
You can choose either the lightbox or embedded option. The "embed" option is probably the style you are most familiar with seeing on the internet. After completing setup, you will see a screen like this:
You'll want to copy those two Site Keys on the right and put them into that captcha configuration dialog on your concrete5 site.
Now your site can use a more human captcha!
All blocks with captcha functionality (forms etc), should be using this new captcha by default. You may need to clean your site's cache if captchas are not working. If a particular add-on is not compatible with this captcha, please contact that add-on's developer for guidance.