Cook up something you like (TechIMO), slap an extension on it (.com), then register the domain name. Several registration services are available.
One example:
http://www.whois.net
Once registered, you then you simply point the domain to the IP address of your Internet server or host.
Domain names are cheap, but you do have to renew periodically.
Hosting can be expensive, but accounts under $20/month can be found for personal hosting. Some even point domains towards free host services, like those often provided by your Internet Service Provider. Many ISP's offer small hosting accounts these days.
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Now if you want to create an entire new domain extension, then you will need to file documents with ICANN. The board will review your request, then determine the actual need for the new extension. While it is supposed to be an open process, the act of developing new extensions can be quite political and very business oriented.
More info:
http://www.icann.org/
Robert Richmond