After your download the VirtualBox install package and install it (just defualt setting).
Then you should download the Ubuntu.iso file from the website.
There is many articles about "do".
What I want to point out are the error when installing Ubuntu:
I saw many questions about this error. You could run into the different case, my solution could not be useful to you and if you had no idea that you could 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 the different disk, for exmaple F. Then the problem is solved.
So, with all, 1. Try to download a different ISO file; 2. Put into different disk.
Help can help to solve the problem.
[VirtualBox] Install Ubuntu 14.10 Error 5 input/output error