ACF in onepage site

I want to use Advanced Custom Fields in my onepage site. Here’s my code in index.php to list all pages:

<?php
                    $args = array(
                        'sort_order' => 'ASC',
                        'sort_column' => 'menu_order', //post_title
                        'hierarchical' => 1,
                        'exclude' => '',
                        'child_of' => 0,
                        'parent' => -1,
                        'exclude_tree' => '',
                        'number' => '',
                        'offset' => 0,
                        'post_type' => 'page',
                        'post_status' => 'publish'
                    );
                    $pages = get_pages($args);
                    //start loop
                    $count = 0;
                    foreach ($pages as $page_data) {
                    $count++;
                        $content = apply_filters('the_content', $page_data->post_content);
                        $title = $page_data->post_title;
                        $slug = $page_data->post_name;
                        $thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'bg' );
                    ?>

I want to create some variable called $subtitle and get my ACF-Field ‘subtitle’. How can I achieve this?