It is very easy to create a bootable Ubuntu Live USB. You just need UNetbootin, Ubuntu.iso image and a USB device.
UNetbootin is a tool, allows you to create bootable Live USB drives for a variety of Linux distributions.
1.Install UNetbootin, install it in Windows or Linux.
2. Download ubuntu.iso file from www.ubuntu.com.
3.Connect USB device and then open UNetbootin (Go to Applications-> System Tools-> UNetbootin).
Check Diskimag, select the .iso file by clicking on marked button at right.Select USB drive at bottom.
Click on OK to start,
After a few minutes, you will see this window.
Now you have this Live USB created.
Just, reboot from USB and in a graphic interface select default.