Forgot the maybe most important piece of information:-)
Download VirtualBox here: virtualbox.org
Don't forget to download and install the Extension Pack from the same page. Latest version is 4.1 and it seems stable to me. Been using it since day one (and the beta before that). Not seen any issues.
Download pre-configured Oracle Virtual Machines from here: http://www.oracle.com/technetwork/community/developer-vm/index.html
Oracle's Virtualization page on OTN: http://www.oracle.com/us/technologies/virtualization/index.html
Generic Virtual Machines can be found here: virtualboxes.org