Domain names you CAN purchase from any register. I recommend GODADDY simply because 'they play nice' with other platforms.
For example, when you use G-Suite for email and Leadpages to build your website, they have an automatic connection during setup process to connect your Domain Name, Email Address and Website. So you won't have any technical work you're not familiar with. Other Domain registrars you'll be required to set the CNAME, A-Records and MX-Records. 🥴
The search results will show and confirm if this domain is available for purchase. Select 'Add to Cart', followed by 'Continue to Cart' next to the search bar.
You'll be taken to an upsell page. Select no, untick everything and continue to cart.
Create an Account and Go through the checkout process to complete the domain purchase.
Once you have gone through the checkout process, GODADDY will temp you to use their Website and Email tools. You wont need these, you can skip.
On your dashboard, under Manage Domains, Make sure auto-renew is switched on for automatic annual billing. This avoids the risk of losing your domain if you forget to renew.
Congratulations! 🍾 Thats the domain purchased... you can now logout, you don't need to do anything else in GODADDY.