We installed Oracle VirtualBox and installed Ubuntu Server 12.04 on it. This youTube video will guide you through the steps of the installation, with a few modifications to the instructions.
Choose VDI (VirtualBox Disk Image) for the virtual file type, skip the part about setting up the network on the VM (it won't ask you to configure network interface during the installation). Do not encrypt your home directory, and when asked about Partition Disks, choose "Guided - user entire disk" (LVM makes it more difficult in the future). When the screen comes up for Software Selection, don't choose any of the software installations, we'll do this all manually during the semester to let you get familiar with the linux package manager. You can stop watching once you boot into the linux system (don't set up any network interfaces)