If you already have a web host, obtain from them the names of their primary and secondary nameservers. Don't worry if you don't understand what these things mean. Just save the information somewhere. The information can usually be obtained from their FAQs or other documentation on their site, usually under a category like "domain name" or "DNS" or "domain name transfer" and the like. If you can't find it, email them. You'll need the information to point your domain name to your website after you buy your domain. Having said that, if you don't have a web host yet, don't worry. Just read on.
The right to use a domain name is delegated by domain name registrars, which are accredited by the Internet Corporation for Assigned Names and Numbers (ICANN), the organization charged with overseeing the name and number systems of the Internet. In addition to ICANN, each top-level domain (TLD) is maintained and serviced technically by an administrative organization operating a registry. A registry is responsible for maintaining the database of names registered within the TLD it administers. The registry receives registration information from each domain name registrar authorized to assign names in the corresponding TLD and publishes the information using a special service, the WHOIS protocol.
If you do not have a web host, you can always allow the registrar to park your domain name at a temporary website specially set up for you. This way you can quickly secure your domain name before it's too late and still take your time to set up the other aspects of your site. As far as I can tell, most registrars (or probably all) automatically park your domain by default whether you ask them to or not, so if this is your situation, you probably don't have to do anything special to get it done. Some of those registrars also provide you with a free email address at your own domain name while it is parked at their site, like email@example.com (where "example.com" is your domain).
When done right, the domain trade is an enterprise that lends itself to particularly lucrative deals. The tactic is simple: domain names are purchased with the prospect of reselling them for a wide profit margin. But the trick of this trade lies in securing domains that may later be valuable to well-endowed buyers, such as a large company. We have laid out all the important facts and terms on the topic, including some of the highest sale prices on record for a publicly traded .com domain.
Registrars offer a wide variety of registration durations—one year, three, five, and even ten. Be careful about registering for more than a year, though. First, there might be restrictions on your ability to transfer the domain name should the registrar give poor service. Second, the registrar could go out of business, leaving your domain name without a host. Check the policies closely.