1) You need to register a domain name
2) You need to get a permanent internet connection
3) Your ISP will give you an IP address and some addresses of it default dns servers
4) Then you need to map your domain name to your IP address on an internet dns server. (Your ISP can usually help with that).
Then you need to design a network locally (at your place) because that will determine how you configure your web server. (Eg Will you put it behind a NAT gateway or just stick it out there in front of everyone?) If you do the latter, you simply give your web server the ip address and domain suffix that you already mapped together on the internet dns server somewhere. You also give it the dns addresses your ISP gave you (they help it to join the internet whereas the other dns server that you mapped your name to your address on helps other people to find you) . Its customary, but not essential, to give your web server the hostname "www".
In practise, I wouldn't do it that way. I would always put it behind a firewall gateway with NAT operating. Even so the same general principles apply.