You need a startup disk or USB flash drive loaded with Ubuntu to install it on a new computer. Ubuntu can be downloaded as an ISO file. The Startup Disk Creator tool can be used to create such a startup disk from a ISO file.
Startup Disk Creator will be present on a Ubuntu computer. If by chance you do not have it, it can be installed easily:
$ sudo apt install usb-creator-gtk
Download the Ubuntu (or any other) ISO file you need to make a startup disk from.
Insert the USB stick or drive that you want to use as the startup disk. Note that it needs to be in FAT or FAT32 format! (Otherwise Startup Disk Creator will not show this drive.)
Start the Startup Disk Creator from the Dash or
usb-creator-gtk from the shell.
Choose the source disk image (the downloaded ISO file) and the disk to use and click Make Startup Disk.
Once it is done, you can use this USB flash drive to install Ubuntu on other computers by booting from this drive! 😃
Note: An alternative to Startup Disk Creator is UNetBootIn, which can be used as described here. I have faced problems using it for making startup USB flash drives of Ubuntu 14.10.
Tried with: Startup Disk Creator 0.2.56 and Ubuntu 14.04