Updated
You can start with a name before you have a website. First understand what you are buying and which service you will connect later.
One address, several uses
In https://www.example.com/contact, example.com is the domain, www is a subdomain and /contact identifies a website page. The full address is a URL.
A domain can serve a website, email or both. Registration by itself contains neither web pages nor an inbox.
Choose each service separately
Registration maintains the name for the agreed term. Hosting stores and serves website files. Email services operate your mailboxes. DNS connects the name to your chosen services.
Keep a website at its current host and connect your domain to it. A domain purchase does not require website hosting. A mailbox remains a separate purchase.
Before choosing your name
Ask someone else to read and say the name. Try it in an email address and check accented spelling. Accents can produce a different technical form; compare both displayed forms before confirming.
A search reserves nothing. An available result still needs checking during checkout; registration is secured after successful completion is confirmed.
Keep control over time
Choose the right holder at the start: yourself, your company or your association. Keep a reachable contact address, including if the domain develops a problem.
Record the expiry date and renewal price. You can later change DNS connections as your project develops without changing its name.