php file and wordpress database

I have a WordPress blog and would like to add a php file to it that interacts with the database. I am a novice(obviously) so I’m not sure what to do once I copy the PHP code in a notepad and save it as something.php

I know the WordPress blog has a SQL database and I have cPanel offered by my host, so I ask for a quick point form explanation on how to generally make the sql database work with the php code? In other words, do I just upload the php file to the root directory and add a line of code (for instance: require_once(“something.php”); to the index.php file ?

Thank you very much.