Turn the computer off in the normal way.
Turn the computer on, tapping the F2 key [it might not be F2, could be esc, F1 F8 or F10, depends on the make and specific model of your PC, for my HP desktops it's F2] a few times each second. This should enable you to enter the BIOS set-up (which key to press) (see https://answers.yahoo.com/question/index;_ylt=A0LEV2bavtZTVmcAjVFLBQx.;_ylu=X3oDMTE0bTVyc3Q3BHNlYwNzcgRwb3MDMQRjb2xvA2JmMQR2dGlkA1ZJUFVLMDdfMQ--?qid=20071012195523AAXHNK3
Look at he screen carefully while you do this, because the computer often tells you how to enter the BIOS setup during the start-up process, but the information isn't the for very long: you may have noticed this while you starting the computer all those times in the past.
Once in the BIOS set-up navigate to BOOT options or something like that. You may have to work your way every option (there aren't that many) to find it. Once there it will show a list of places to boot from - these are typically the hard drive, the optical disk drive, and the USB port, you may even have eSATA option. These will be in order. Change the order to make the laptop try to boot from a USB device first. Now exit the BIOS set-up saving the change to the boot device order. If there isn't a USB device when you next boot the laptop will just try the second device on the list. If you start up the computer with a bootable USB device in a USB port, it will boot from that device. (eg a flash memory drive)
There are other ways, but this is fundamental way if your BIOS supports boot from USB (my desk tops do not so I had to find another way)