When you select a virtual machine from the list in the main VirtualBox window, you will see a summary of that machine's settings on the right of the window, under the "Details" tab.
Clicking on the "Settings" button in the toolbar at the top of VirtualBox main window brings up a detailed window where you can configure many of the properties of the VM that is currently selected. But be careful: even though it is possible to change all VM settings after installing a guest operating system, certain changes might prevent a guest operating system from functioning correctly if done after installation.
Note
The "Settings" button is disabled while a VM is either in the "running" or "saved" state. This is simply because the settings dialog allows you to change fundamental characteristics of the virtual computer that is created for your guest operating system, and this operating system may not take it well when, for example, half of its memory is taken away from under its feet. As a result, if the "Settings" button is disabled, shut down the current VM first.
VirtualBox provides a plethora of parameters that can be changed for a virtual machine. The various settings that can be changed in the "Settings" window are described in detail in Sectioná3.7, “Virtual machine settings”. Even more parameters are available with the command line interface; see Chapterá8, VBoxManage reference.
For now, if you have just created an empty VM, you will probably be most interested in the settings presented by the "CD/DVD-ROM" section if want to make a CD-ROM or a DVD-ROM available the first time you start it, in order to install your guest operating system.
For this, you have two options:
If you have actual CD or DVD media from which you want to install your guest operating system (e.g. in the case of a Windows installation CD or DVD), put the media into your host's CD or DVD drive.
Then, in the settings dialog, go to the "CD/DVD-ROM" section and select "Host drive" with the correct drive letter (or, in the case of a Linux host, device file).
This will allow your VM to access the media in your host drive, and you can proceed to install from there.
If you have downloaded installation media from the Internet in the form of an ISO image file (most probably in the case of a Linux distribution), you would normally burn this file to an empty CD or DVD and proceed as just described. With VirtualBox however, you can skip this step and mount the ISO file directly. VirtualBox will then present this file as a CD or DVD-ROM drive to the virtual machine, much like it does with virtual hard disk images.
In this case, in the settings dialog, go to the "CD/DVD-ROM" section and select "ISO image file". This brings up the Virtual Disk Image Manager, where you perform the following steps:
Press the "Add" button to add your ISO file to the list of registered images. This will present an ordinary file dialog that allows you to find your ISO file on your host machine.
Back to the manager window, select the ISO file that you just added and press the "Select" button. This selects the ISO file for your VM.
The Virtual Disk Image Manager is described in detail in Sectioná3.5, “The Virtual Disk Manager”.