The Internet root domain, top-level domains (organizational and geographical) are maintained by the various Internet governing bodies. People with networks of any size can "join" the Internet by registering their domain name in either the organizational or the geographical hierarchy.
Every DNS domain must have a domain name. If your site wants to use DNS for name service without connecting to the Internet, you can use any name your organization wants for its your domains and subdomains, if applicable. However, if your site plans wants to join the Internet, it must register its domain name with the Internet governing bodies.
To join the Internet, you have to:
Obtain a network IP address from that governing body.
There are two ways to accomplish this:
You can communicate directly with the appropriate Internet governing body or their agent. In the United States, InterNIC is the company that currently handles network address and domain registration matters.
You can contract with an Internet Service Provider (ISP) to assist you. ISPs provide a wide range of services from consulting to actually hosting your Internet presence.