Before you can get a domain, you have to choose a registrar. Different registrars offer different pricing, can sell different TLDs (such as .com, .org, and so forth), and have different features and capabilities.
Our favorite registrar is NameSilo. It offers a huge variety of TLDs at extremely competitive prices. It also doesn’t charge extra for basic features we believe should be free and universal, such as privacy protection.
We also use Google Domains and Namecheap sometimes, as well as national registrars when we need a domain only offered by a national registrar (such as .es or .im).
We try to avoid registrars that have unpredictable pricing, endless upsells, or questionable ethics. Unfortunately, there are quite a few of those, including some very big names.