Create the Forest Root Domain
By default, the first domain created in your forest becomes the forest root domain. You have the choice to keep the existing domain as a forest root domain or to create a dedicated forest root domain in a multi-domain scenario. The forest root domain must be managed centrally in an organization because it provides the foundation of the Active Directory forest infrastructure. It is responsible for policy decisions, domain hierarchy, and naming.
If you have a single domain forest then the forest root domain will contain all the users, groups, and resources. However, if you have a multiple domain forest and you create a dedicated forest root domain, then it will contain only forest administration accounts.
The first step in the creation of a forest root domain is to install and configure the first root domain controller. The first domain controller in the domain automatically take all of the five FSMO (flexible single master operations roles) roles and becomes the first global catalog and root domain controller.
Follow the steps 1-13 in the Install and Configure Active Directory Domain Controller section to create the first root domain.
After installing the first root domain controller, you need to verify the domain installation, DNS installation, DNS integration with AD, installation of shared system volume, and operation directory services restore mode boot option. You can perform all these verifications by selecting the appropriate Administration Tools.