How to Reserve a Domain Name (The Detailed Answer)
A website is a separate thing from a domain name. As I explained in my article on How to Create a Website, a domain name is just a name. When you register a domain, you merely get the right to associate it with your website. In the brick and mortar world, a domain name is like the business name that you register with some government organisation ("organization" in US English), while a website is analogous with your shop or office premises. Just as you can register a business name without renting premises in some building for your shop, so also can you register a domain without "renting" space on a web host for your website files.

Having said that, most (if not all) domain name registrars, the companies through which you get your domain, will automatically associate a dummy (placeholder) website with your domain when you register it. This is called domain name parking, and the end result is that when you type your domain name into your browser, you end up at the placeholder site set up by your domain name registrar. See my article on What Does It Mean to Park a Domain Name? Domain Name Parking Explained for more information if you want to know more about this.

What if you get a registrar that doesn't automatically park your domain name? Well, it actually doesn't matter. Don't believe the advertising hype (if any) from a registrar that implies parking is of any importance at all. In my opinion, this kind of domain name parking doesn't really benefit you at all: it benefits the registrars since they dump their advertisements on that placeholder site. The fact of the matter is, even if your domain is not parked or anything like that, once you've registered it (and paid for it), it's yours to do as you like. You can put a site on it immediately, or if you prefer, at some later date. No one else can buy it and put up their sites on that domain for as long as you own that domain.

(For the curious, all that happens when you don't associate any website with a domain is that when someone types that name in a web browser, the browser will tell that person that the website does not exist or something like that. Don't worry. This message, in such a situation, merely means that the domain has no associated website; it doesn't mean that you suddenly lost your domain or anything of the sort. If you've paid for your domain, and kept your domain registration up to date by renewing it, you still "own" that domain no matter what the browser says. Remember that browsers don't really care about whether someone has registered a domain or not; their job is to display the websites located at those domains, not to give you a report on the state of the internet.)

In summary then, to reserve a domain name, you simply have to register it the normal way. The exact procedure can be found in the article How to Register a Domain Name, and I won't repeat it here since you can just click through to read it. That's all there is to it. Since you are not going to start a website immediately, your job is done. (But remember to renew the domain every year if you want to continue holding onto the name.)