Click New to make a new virtual machine.
Name it and select Linux for the operating system , version Ubuntu.
Select how much memory you'd like.
Create new virtual drive around 15-20 GB is good .... click Finish
Before you start up the VM go to settings storage
The IDE Controller has to be linked to the ubuntu.iso so it can boot that iso and you can install it onto the virtual drive you made earlier.
Some tips...
You'll need 3d acceleration turned on, it is located in the settings >>Display >> Video >> click the "Enable 3d Acceleration" and also give the "Video Memory" around 128mb for the Ubuntu Unity desktop environment to work.
If you have a multi core CPU boost the number of processers to 2
Command to share a folder in linux guest is (open a terminal)
sudo mount -t vboxsf Documents /home/USERNAME/Shared
Where Documents is the name of my windows folder I shared in the Virtualbox Settings
USERNAME = the name you chose
/home/USERNAME/Shared is a folder named Shared in my home folder
If you get lost with Ubuntu and Unity, try Mint with the Cinnamon desktop, it is my preferred linux distro at the moment