Install VirtualBox, then hit New. Choose Ubuntu, and then pick what size RAM and hard drive you want (it's all virtual, there's no actual partitions). I originally chose 2 GB and 20 GB, respectively, but I think people have chosen way less and been fine. (It is a quite a process to change the hard drive size later on if you need to, but I'll save you the gory details here on how. Just know that it can be done.)