In Drupal, JavaScript files are added via the drupal_add_js() function or in the
theme’s .info file. 
A simple example of using jQuery in drupal is

        <?php
        drupal_add_js('jQuery(document).ready(function () {
       jQuery.ajax({
            url: '/code/test-ajax.php',
            data: {'a':'hello','b':'world'},
            type: 'POST',
            success: function(data){
                alert(data); //Should return "hello, world"
            },
            error: function(jqXHR, textStatus, errorThrown){
                alert(textStatus + errorThrown); //Better diagnostics
            }
        });
        });', 'inline');
        ?>

you have to use 'jQuery' instead of $.