Here are three stumbling stones I came accross while installing Ubuntu 14.04 as a guest on a Windows 8.1 VirtualBox host system:
1) To be able to install Ubuntu 14.04 as 64-bit I needed to go to the BIOS settings and enable virtualization
2) After the installation the screen resolution was awfully small and could not be changed. I had to install the "Virtual Guest Additions": sudo apt-get install virtualbox-guest-dkms
3) To be able to access an automatically mounted shared drive you have to add your ubuntu user to the group "vboxsf". Then you will be able to see the shared drive 'sf_*' under '/media', e.g. '/media/sf_mySharedFolder'