Plug the Live USB into the PC and then turn it on.
If you aren't greeted with an Ubuntu screen showing "Try Ubuntu" or "Install Ubuntu"
click on Troubleshooting Starting a Live USB
Choose "Try Ubuntu" you should get a desktop after a few mins.
If you're stuck at a black screen, click on Black Screen After Starting a Live USB
After the desktop has loaded you can click on "Install Ubuntu" or you can use the program "Gparted" in order to make changes to your hard disk partitioning.
If you press Ctrl + Alt + T
you'll open a terminal and should see a command prompt such as ubuntu@ubuntu:~$