Need help with sending email via custom module

I’m new to Drupal and trying to write a simple module as a method of learning this CMS.

On one of the pages I want user to click a button which will send email to site admin. That email must contain text of the page it is sent from.

Page is build via hook_menu()

function shop_menu() {
	$items = array();
	$items['cart'] = array(
		'title' => 'Cart',
		'page callback' => 'shop_output',
		'access callback' => 'user_access',
		'access arguments' => array('access content'),
	return $items;

And here’s shop_output() function

function shop_output() {
	$cart_content = 'Content of the page';

	return $cart_content;

How do I implement such button on that page? Or maybe I’m wrong and it should be done some other way?

Drupal version: