If you have not been working with Management Groups to manage Azure at scale, I recommend to review the documentation. You should also take a look at the Microsoft Cloud Adoption Framework (which was just recently updated).

When you start using Management Groups you might want to change the display name of the root management group or do other changes to the hierarchy. To get started you need to have access to the root management group. If you have global administrator permissions, follow the documentation (especially this) on how to set that up.