Choose memory size (I chose 2048M RAM, for some JetBrains products like IntelliJ and CLion, 2GB is minimum and 4GB is recommended. Note that your hosting machine = your computer needs a minimum of RAM to function and for running the virtualbox system itself... )
Hard Disk: choose "Create a virtual har disk now", the size depends on the free space on your computer but at least 8GB
the type of the disk image can be VDI (to use with oracle virtualbox), VHD (can be use with windows virtualization as well) or VMDK (can be used with VMware virtualization)
For the installation, attach the ISO file to the virtual machine:
Click on the new virtual machine you created
Click on settings, choose "storage"
Click on the cd image on the right of the "Optical Drive" and "Choose Virtual Disk File...", choose the debian iso file from your Downloads directory.
Run the machine, and debian installation will start.
Choose language (I recommend to use English as installation language, and later choose an Hebrew keyboard.)
Choose all default values, root password as you wish - but something you'll remember when you'll have to run commands with root (administrator) permissions.
In Disk Partitioning you'll have to confirm the partitioning, choosing "yes" instead of the default "no"
Right after the basic installation you would be asked to choose additional software. I recommend installing "GNOME" or "KDE" desktops, since JetBrains solutions mention it as system requirement. if you want interface similar to CS', install MATE.