After you download the VirtualBox install package and install it (just defualt setting).
Then you should download the ubuntu.iso file from the website.
There are many articles about how to do this.
What I want to point out is the error when installing ubuntu:
I saw many questions about this error. You may run into different case, my solution may not useful to you, but if you have no idea, you can try as I did.
I want to write down how I solved this problem (actually even cannot say I "solved"...):
1. At first I download the ubuntu version:
Ubuntu 14.04.1 LTS
put this iso file in C disk. But when installing, I got the error.
2. Then I try another iso file:
Ubuntu 14.10
and put into different disk, for exmaple F. Then the problem is solved.
So, in all, 1. try to download a different iso file; 2. put into different disk.
Help can help to solve the problem.