You need to burn a disk containing Ubuntu.
go out to the drug store and buy a pack of five blank DVDR disks.
on your computer :
download the latest version of Ubuntu :
from here : http://cdimage.ubuntu.com/daily-live/current
windows :
go to http://imgburn.com and download the program
insert one of the blank DVDs from the pack you bought at the drug store.
find your Ubuntu ISO, right click on it, and select "burn using img burn".
uncheck the "verify" box
change "aws" to "2x"
click write
wait for the disk to finish burning
boot from it
mac : go to utilities > disk utility
insert one of your blank disks
click the "burn" button
find your iso
click burn
wait for burn to complete
boot from your disk
linux :
open a terminal
insert one of your blank disks
in the terminal,
enter
sudo wodim /path/to/your/ubuntu.iso
and enter your password when prompted.
When your prompt returns, boot from your disk.
When you hear the drum sound, click your language, and then click the "install ubuntu" button.
click through the prompts, until at the "this computer currently contains ___" where ___ is your current operating system. Click the "erase ____ and install Ubuntu" option, and then click continue. Keep on following prompts until your install completes, then remove the disk and click reboot.