Google AdSense is the most popular online marketing tool and is free.
With it you can generate money from third parties.
The received advertisements are related to the content of your site. They are paid according to the type and number of clicks received on them. You are also paid for the number of ad views on your site. If you also include a Google search box in your site code, then you have even more to gain.
However in order to make a decent sum you must have an attractive website and well positioned ads.
The amount you can earn can be seen in your Google account but first you need to sign up: https://www.google.com/adsense/