I realise that you're a beginner, but you must understand that i wrote the Joomla howto as easy as possible. With other words: I can not explain it better than within the howto.
Did you read the manuals at http://www.ispconfig.org
They help you to setup a client and a website with a MySQL database within ISPConfig.
Not Joomla related but, i want to know:
Did you register your domain name at your DNS-provider and is that domain name pointing to the IP-address of your server? If not, change the DNS-records for that domain name.