when you are booting your computer, press F11 to enter your boot screen. Actually, it might not be F11, but for my computer it was. Just try F8-12 and one of those should work. Also, the boot screen is different from the BIOS, so if you're in the BIOS, you're not in the right place.
Anyways, once you're in the boot screen, you can select which peripheral to boot from.
Alternatively, you could change the order of preference when attempting to boot. By default, most systems check the CD/DVD first if no OS is found on the HDD. If you change the order to your USB drive, you should be able to boot from a flash drive with no issues.