Implementation of hook_block()

I found this code online and I would like to implement it. However, I have never worked with hook functions.

My question is when I put this code into a brand new php file ex: uc_microcartTest.php . How do I call this new php file and get the results to show like this?

/**
 * Implementation of hook_block().
 */
function uc_microcart_block($op = 'list', $delta = 0, $edit = array()) {
  switch ($op) {
    case 'list':
      $blocks[0] = array(
        'info' => t('Micro-sized cart block for page header.'),
        // This block cannot be cached, because anonymous
        // sessions can have differing cart contents.
        // To improve this, see drupal.org/project/uc_ajax_cart
        'cache' => BLOCK_NO_CACHE,
      );
      return $blocks;
    case 'view':
      if ($item_count = uc_cart_get_total_qty()) {
        $block = array();
        $block['subject'] = '';
        $block['content'] = theme('image',
          drupal_get_path('module', 'uc_cart') .'/images/cart-full.png');
        $block['content'] .= format_plural($item_count,
          'My cart: 1 item', 'My cart: @count items');
        $block['content'] = l($block['content'], 'cart', array('html' => TRUE));
        return $block;
      }
      break;
    }
}

Although it is possible to call the functions from external PHP files but I recommend you follow the "Drupal way":

1. Create your own custom module
2. Implement hook functions in <your_module_name>.module file.
3. Enable your module

You should also read more about how hooks work in Drupal here