This is really more of question for whomever runs your network and external websites. Essentially, they will need to provision an external domain name like "myTableauServer.myCorp.com" and have connections to the IP address associated with the domain routed to the Tableau Server running on your computer. This process is all about networking, firewalls, etc. and has little to do with Tableau. The question you need answered is "How do I expose a web application (in this case, Tableau) running on an internal server to the outside world?"