In order to make an Ubuntu live USB for installing Ubuntu, use UNetbootin (Windows/Mac/Linux/Ubuntu Software Center). The flash drive you use should be 1GB or larger (2GB or larger for Ubuntu 14.04 and onwards), plus whatever additional space on the USB drive you want to use for persistent file storage, and formatted to FAT32. UNetbootin will automatically format the USB flash drive to FAT32 if it is not already formatted to FAT32. In either case the formatting procedure will delete all of the files that are already on the flash drive.
It is very important to verify that the device that you are installing the Ubuntu live USB to is indeed your flash drive, so that you don't overwrite any of your system or personal files which may make your operating system unbootable. In Ubuntu you can find the device name of the flash drive using Disk Utility (or Disks if you are using Ubuntu 13.04, 13.10 or 14.04). In the picture below the name of the flash drive is
/dev/sdc1 where /dev/sdc is the device name of the flash drive and the 1 after sdc means UNetbootin will install the Ubuntu live USB on the first partition on that device.
The Ubuntu live USB flash drive that you make this way will be bootable from PC computers. In order to enable your computer to boot from the USB flash drive, you must enter your BIOS menu by pressing one of these keys, usually: Delete, F2 or F10 as soon as the motherboard splash screen appears when the computer is booting. In the BIOS menu, you need to change the boot order so that the USB flash drive, which is usually called USB-HDD in newer computers, is the first entry in the boot sequence, and then save your new BIOS settings and exit the BIOS setup.