connecting php script with wordpress API

Hey guys!

I was wondering how to get a connection to the wordpress API from an external script.

In the root directory I have a php script which needs to access WP to read page contents.

To get the content of a page (what the php script is supposed to do) I’ve found the following thread on the forums:
https://wordpress.org/support/topic/how-display-content-on-a-custom-page?replies=9

But which files need to be included to use, for example, get_header()?
My script is not a template, at least I think so.

I’ve also found an article for wordpress includes (https://codex.wordpress.org/Include_Tags), but it still doesn’t help, as long there is no connection to the WP system.

This might be a nooby question, but my research ended in no solution.
I hope you can help me,
Regards!

[ No bumping please. ]