Hello @mic and welcome to the community.
It is essential that you install Virtualmin via Automated Virtualmin Installation – Virtualmin. This enables the community to assist you effectively and it is also the easiest way to install Virtualmin correctly.
A new top level virtual server for each domain. A new virtual server can be created via Virtualmin → Create Virtual Server
I don’t use sub servers unless there is good reason to do so. You could avoid sub servers in the beginning of your journey with Virtualmin to keep things simple. Later, if you wish to convert a top level virtual server to a sub server, you can easily do that (and vice versa) via menu options in the Virtualmin web interface.
This is the link to Virtualmin official documentation
https://www.virtualmin.com/documentation/
This forum has a search feature
https://forum.virtualmin.com/search
Additionally, feel free to ask questions. Please take a quick look at the community guidelines before you engage.