Here, I'll explain how to install Ubuntu Linux as a guest OS on Debian 9.0 Stretch Linux using VirtualBox.
Here are the specs of the Debian host OS:
command: lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 9.0 (stretch)
1) Install VirtualBox: how to install virtualbox
2) Set up your guest Ubuntu-OS. Launch VirtualBox
a) Click New: the Create Virtual Machine Window Opens
Name: Ubuntu<description here>
Version: Select either 32-bit / 64-bit
b) Click Next: Memory size...use the scrollbar for a memory setting, depending on your available capacity. I usually do 1024MB
c) Hard disk = Create a virtual hard disk now
Hard disk file type: Leave at default VDI (virtual box disk image)
d) The Hard disk file type window option opens(below)
Default option is VDI. Leave as is
e) Storage on Physical Hard Disk
Leave at default: Dynamically allocated
f) File location and size: use the scrollbar to select your Virtual OS HD capacity. 8-10Gb is a good size
Up till here, the virtual machine's settings are completed, but we havent installed it.
Head on to download Ubuntu. Here are some links:
Back to Virtual Box
Click your created server, and click Start
Use the folder icon to select your iso image, and click start
Follow the installation process.