Can you also tell me what to select when I create a new virtual machine?
For example I picked Debian as OS for Knoppix: was that correct?
Yes Knoppix is based on Debian. But it's a good idea to have enough RAM available for the virtual guest - 33 - 50% of the RAM within your host. My computer has 4 GB RAM. Therefore I use 2048 MB RAM available for the VirtualBox.

You can change this value everytime before starting the VirtualBox session.