Example hook_theme() with template

gistfile1.php

<?php

/**
 * Implements hook_theme().
 * Registers all theme hooks defined by this module.
 */
function mymodule_theme() {
  return array(
    'mymodule_my_hello' => array(
      'variables' => array(
        'title' => NULL,
        'template' => 'mymodule-hello',
      ),
    ),
}


?>