I've produced personal files known as, nl.php within my wordpress theme folder so when I attempt to call a function like wordpress_header or any function, i recieve a mistake message, the function doesn't exist.

How do i solve this issue.

you have to load the wordpress-load.php file, that will then permit you to call wordpress functions.

For instance:

require( '../wordpress/wp-load.php' );

with 'wordpress' being your install root.

You have to load the core Wordpress files.

<?php require 'wordpresspath/wp-blog-header.php'; ?>