We recommend GoDaddy based on their amazing customer service, but you can also use any other domain provider to purchase your domain.
Key tips:
> The moment you know you want it, BUY IT. Bots crawl the internet nowadays and look through your searches. They WILL buy your domain name if you wait on it. It's happened to a few clients, and it'll happen to more. It'll happen to you if you drag your feet!
> Get the standard privacy protection. It keeps your personal information hidden from people on the internet. It also ensures that your domain expiration date is not available. Again, just more help with bots.
> You do not need to buy anything else or sign up for anything else right away. Just get that domain name and sit on it until your web design is done.