forum.opsi.org

We have a lot of computers at my home and I want to try use opsi so I can easy control so all computers has the latest application version.
But, I am not a expert on computers yet so I need some help.

I want to test opsi first with VMWare. In the manual I can read that I need a valid DNS domain and hostname. How I get that?

You have slightly misread the manual there. What you need is a valid domain name. For example, the hostname of opsi.example.org would be opsi, the domain name would be example.org.
Since you probably don't have an enterprise grade DNS in place, I'd assume this is nothing you need to concern yourself with that much.
Keep in mind, however, that opsi is an enterprise solution and as such relies heavily on enterprise grade technology like a working DNS resolution. If you are planing to use opsi in a somewhat reliable environment, you should familiarize yourself with these technologies first.