Create a virtual machine

New VM wizard, controllers, and the first ISO.

Click New, name the VM, pick the OS type, and give it RAM and a VDI disk. Attach the installer ISO to the virtual optical drive. If the guest will not boot, confirm the firmware setting (ISO vs EFI) and that the ISO actually attached.

After the OS is in, install Guest Additions from the Devices menu. That gives you a usable display, shared clipboard, and better mouse integration.