First create an SMTP profile under Administration > Plugins > SMTP Setup. Then create an order confirmation email template under CRM > Email Autoresponders > Templates, by selecting it under the template dropdown and modifying it to your needs. Then create an event under CRM > Email Autoresponders > Events and select the template created above, the SMTP profile created earlier, and the product(s) you want the email assigned to. Finally, apply the events to your site within your site's settings page under CRM menu CRM > Sites.