gain access to core WordPress functions

i created a wordpress plugin but they sent me back some problems i must repair.
i used

<?php  
include '../../../wp-load.php';
?>  

to gain access to core WordPress, byt they told me:

It’s best if you tie your processing functions into an action hook,
such as “init” or “admin_init”.

but i found no good information on the web. i tried this:

<?php  
add_action('get_header', 'my_function_init');
?>

and other things i found on the web but nothing works like wp-load.php.

any advice ?