Nope, you don’t have to install anything, it’s enabled by default. Just visit http://YOURDOMAIN/wp-json/wp/v2/posts, for example, to see it working 🙂
As said it is all new for me and now I have a good starting point. Thanks for helping!
You probably already know it, but here is a link that helped me A LOT some time ago: https://developer.wordpress.org/rest-api/
Also, if your question has been answered, we would love if you would mark this topic as resolved in the left hand sidebar. This helps our volunteers find the topics that still need attention and more people will get helped, possibly like you did.
Thanks again! And marked as resolved 😉