Insert a USB stick with at least 2GB of free space
The easiest way to get up an running with USB is to use the USB installer provided by pendrivelinux.com. You’ll need to download and install that and then come back here and follow the instructions.

Download : Universal-USB-Installer-

  1. Select Ubuntu Desktop Edition from the dropdown list
  2. Click ‘Browse’ and open the downloaded ISO file
  3. Choose the USB drive and click ‘Create’

Sumber : http://www.ubuntu.com/download/server/download