A cornerstone to learning how to purchase a domain name is understanding who sells them.
Domain registrars sell domains. When registering or buying a web domain, you do it through a domain registrar. One of the oldest and most well-known registrars is GoDaddy.
Others include:
Domain.com
Google Domains
Bluehost
Network Solutions
Namecheap
And more!
Instead of using one of the above registrars, you can also start the process of purchasing a website domain through your content management system (CMS), if you use one. In that situation, your CMS — WordPress, for example — will handle the registration for you.