Sure, you could install Virtualbox from your package manager, but then you'd be missing the latest and greatest in updates to Virtualbox. To get the latest version of Virtualbox with updates, here's what you need to do!
1.) Download and add the Sun public key by opening a terminal and running this command:
wget -q http://download.virtualbox.org/virtualbox/debian/sun_vbox.asc -O- | sudo apt-key add -
2.) Add the repository to your Software Sources go to System > Administration > Software Sources > Other Software and then click the ADD button. Enter the line below into the Apt-line then click Add Source.
Ubuntu 9.04 Users (this is not a command):
deb http://download.virtualbox.org/virtualbox/debian jaunty non-free
Ubuntu 9.10 Users (this is not a command):
deb http://download.virtualbox.org/virtualbox/debian karmic non-free
It will probably tell you the packages are out of date and to reload them, go ahead and do that. If it doesn't ask you to reload then you will need to refresh package index so you get the correct version when you install, to do so, run this command:
sudo apt-get update
You can now install the latest version of VirtualBox:
sudo apt-get install virtualbox-3.0
(Double check what the latest version is and modify the command above if necessary. You only need to put 3.0 not 3.0.8, it's not that picky.)
Here is the official documentation on how to create and run virtual machines in Virtualbox:
http://www.virtualbox.org/manual/UserManual.html#id2500706
