Firstly, you search the forums because we've answered this a couple of times just in the last week
2) Check your ISP contract to see if you are permitted to run a server
3) Buy a static IP address or investigate a Dynamic DNS service (do your googling, I'm not gonna type it all out here)
4) Set up port forwarding on your router to your Mac, which has to have a static internal address within your home (assuming you have a broadband connection and a firewall/router device) Read the manuals to learn how.
5) Set up your Mac to do web serving (RTM and Google)
6) Buy a domain name to use, hook it up through your ISP if permitted, or through an outside dynamic DNS service.