Nearly everything depends on the domain name.
The domain name is used for:
- Your Website
- Your E-Mails (behind the @)
- Your internal Active Directory Domain (cannot be changed easily)
Your domain should not contain legal forms like “Corp”, “LLC” or “LLP”, because these often change later on.
Just think about a good name for your website, like “contoso.com” (used for examples on this page).
Next you should check if it is available on e. g. namecheap.com and buy it.
Also If you decided to hoste your services in House, you need to buy some public IP addresses from your provider IPv4 /29 and IPv6 /61 (smaller number is better), if possible with bgp capabilities, but as mentioned before, this is out of scope for this tutorial, as it is a very deep in tech topic and you no matter what require multiple IT Specialists to plan, setup and maintain this setup.