How to echo "Hello World" in civicrm?
The civicrm_post hook performs stuff after a database update, and does not necessarily just echo stuff on a page.

If you want to customize CiviCRM I strongly recommend you read the Developer Guide carefully, and possibly check if you can follow a CiviCRM developer training somewhere.

I assume you do not actually really want to echo "hello world" on any page, so perhaps it will help if you tell us what you really want to do?