A domain name just gives you control over the DNS records that computers use to map a hostname to an IP address. (Connections over the Internet are always from IP address to IP address, not by name alone.) The servers that answer DNS queries won't also host your web site; that's a separate thing.
See other answers for details.
An analogy may help understanding them:
- DNS is like a phone book, mapping names to numbers.
- A phone number is like an IP address
- An answering machine is like a server; if configured to take calls, it will. But it's only reachable if connected to a land-line with a permanent phone number, and people know how to reach it by looking up a convenient name in the phone book to find that phone number.
- A web-hosting service is like an answering service, with answering machines connected to phone lines ready to answer calls. You can pay them to take calls for your site by name, or even with a dedicated phone number (IP address) that isn't shared with any other site.
With just a domain registered, you're paying for an entry in the phone book that maps your name to a phone number. That's all you have so far, no phone number to point it at, and no answering machine that answers calls.
So you have nothing useful to list in your phone-book entry.
This is a fairly decent analogy. Virtual hosting doesn't need every "server" to have a separate IP address; it can sort out who the caller wants to contact by name after they call a shared phone number. (via HTTP headers in the request).
You could list your home phone number, i.e. home desktop computer, if your ISP happens to give you a stable IP address and allows hosting servers. But then you'd have to be responsible for being the admin of your own web server, keeping up with security updates. (In the analogy, using your home answering machine for this, making it the target of crank calls and spam, and people who fill up the tape, or try to break it.)
Many ISPs don't give you a stable IP address with port 80 (http) or 443 (https) open, or have rules against actually hosting a web server. But even if you technically could, with your level of experience you wouldn't want to. I mention this only for completeness of the analogy, and to maybe help understand what it is that a web-hoster is actually doing.