AJAX and accessing the WP database.

I am using a custom template that after the ‘the_excerpt()’ call I am ‘including’ another php file. The php file has 2 div tags side by side, the left div has a map image and the right div tag is empty. In the map are HTML hotspots that when moused over call a jQuery request to load another php file, getBranch.php. This second php file has various WP-Query statements and the one that is run will depend on the ‘location’ value sent from the map hotspot, I.E.

jQuery(document).ready(function(){
jQuery("#North").mouseover(function () {
jQuery('#location').load('https://mysite/wp-content/themes/mytheme/getBranch.php?location=north', function() {
/*alert('Load was performed.');*/
});
});
});

The jQuery function is working fine, however, the getBranch.php file has no access to WP_Query so I am unable to run any SQL queries, I simply get “Fatal error: Class ‘WP_Query’ not found”.
I have read many, many articles on this problem and can see the solution, add_action etc, but cannot put this all together.
I have tried the ‘sloppy’ way of solving this by including the WP core files, wp-config, wp-load and wp-includes/wp-db but I just get a handful of PHP errors. I am using PHP 5.3.
Can somebody create a ‘step by step’ example of how to do this properly including which code goes into which files please?