I would say that you need to first format your USB stick (Insert it and go to My Computer, right click USB stick icon and choose Format). There are some options in the format window. One of them is to make the USB stick bootable. Tick this box before you format it. When finished it should boot after a restart. You'll want to go into your Bios settings (delete button at reboot when asked) and have your boot order change to have your USB boot beofre your HDD.