Buying the name is the easy part. Pointing it at your server is where most people get stuck.
A domain name on its own does nothing. It becomes useful when the name servers point at a provider that knows your server’s address.
Set the name servers first
Do this before anything else, because it is the change that takes longest to spread. Somewhere between a few minutes and a day, depending on what the previous setting told the world to remember.

Then the records
- A — the address of your server
- CNAME — one name that follows another
- MX — where your mail goes
Get the A record right and the site loads. The rest can wait until it does.