To create a USB stick from which you can install Ubuntu, you must first download Ubuntu. Then, follow these instructions:

Note: These instructions should work with all varieties of *buntu and derivatives, as long as they can be installed from an ISO file. These are the easiest instructions we’ve seen for doing this, but you need to have a working Ubuntu box (or any *buntu version that includes the Startup Disk Creator software, or on which you can install that software) to create the bootable USB stick.

Full article here:
How to create a bootable USB stick on Ubuntu (Ubuntu.com)