From Joomla! Documentation
- Log on to the back end of your website with an Administrator account.
- Go to Extensions > Module Manager and create a new module instance of module Custom HTML.
- Go to Site > Global Configuration and change the default WYSIWYG editor back to what it was.
There are a lot of extensions for implementing Google AdSense (see the Google Ads section of the Joomla! Extensions Directory. Maybe some of them can be useful if you have specific needs. But if you just want to use AdSense as it is, you don't need them.