A website is a collection of documents that live on a computer somewhere in the world, is connected to the Internet, and has an address assigned to. Individual website domains can be pointed to a specific document (using index.html), within that folder, on that computer connected to the Internet--also called a web server.
A Website Always Begins With A Domain, Your Domain
A website begins with you purchasing a domain name. In the early days most domains ended with .com, but today, there is almost any top level domain you can possibly imagine. Anyone can purchase a domain, I recommend using a domain registrar named Hover, a service that allows you to purchase any available name, and manage all of the detail of where your domain will live.
Where Is The Computer Will Host Your Website(s)?
Each hosting provider will have a different setup for the computers they run, and connect to the cloud. I run Linux servers, within the Amazon Web Services cloud platform. I use an open source web server software called Apache to deliver web sites on my Linux servers. To setup a new website, all I do is create a new folder, add a new entry to the configuration file for Apache Web Server, restart the service, and my basic website is up, and running for potential visitors. However, before I can actually visit my website, I need to setup the DNS, which is the address language that is used route all Internet traffic.
DNS Is How You Connect Your Domain To Your Host
That is it, obviously there are many other very technichal things you can do with a website, but this shows should help you understand that websites are just documents sitting on a computer somewhere that is connected to the Internet. It is up to you to purchase hosting, purchase your omain, and connect your domain and subdomains to that location on the computer.