How to retrieve form data?

You were almost there. I have modified your code. You could try this:

<?php
add_action('admin_menu', 'my_plugin_create_menu');

function my_plugin_create_menu() {

    //create new top-level menu
    add_menu_page( 'My Plugin',
                    'myCust Form',
                    'administrator',
                    'insert-my-plugin_bro',
                    'my_plugin_settings_page',
                    'dashicons-translation',
                    '60'
                );  

    //call register settings function
    add_action( 'admin_init', 'register_my_plugin_settings' );
}


function register_my_plugin_settings() {
    //register our settings
    register_setting( 'my-plugin-settings-group', 'display_name' );
}

function my_plugin_settings_page() {
?>
<div class="wrap">
<h1>My Plugin Settings</h1>

<form method="post" action="options.php">
    <?php settings_fields( 'my-plugin-settings-group' ); ?>
    <?php do_settings_sections( 'my-plugin-settings-group' ); ?>
    <table class="form-table">
        <tr valign="top">
        <th scope="row">Enter a name to display</th>
        <td><input type="text" name="display_name" value="<?php echo esc_attr( get_option('display_name') ); ?>" /></td>
        </tr>
    </table>

    <?php submit_button(); ?>

</form>
</div>
<?php } ?>

WordPress also has a documentation for this . It should help you