Problem in connecting database

I am new babe to WP.
I have created a folder name json in :
http://localhost/my_project_name/wp-content/json/
Inside directory I have made a file jsonquery.php which performs the code to import data from MySql and convert into JSON.

My problem is, how can I connect database in this file. And use WP query function to fetch records?

Thank you for the support.