Upload the PHP Files/Application to your PHP Web server. Create a MariaDB/MySQL Database, and input the database credentials in /php/config.php
Using PHPMyAdmin (or CLI), import the db.sql SQL Database structure into your database which you just created, then delete the db.sql file from the root directory of the application.
You have finished setting up & configuring bluegreen. Now, open it in your browser (if you are running something like IIS or AMPPS on windows, navigate to localhost or 127.0.0.1 in your browser. else, you know where to go.) and register a new account there. This chat application is not designed to be used by millions, or thousands of people. It's designed to allow a bunch of people to privately message each other, all from a common workplace, school, etc.