If it doesn't automatically boot to the drive, you'll need to reboot your computer into its BIOS firmware -- usually done by pressing Esc, F2, or a similar key while the computer is starting up -- and change the boot drive or "bootmenu" to your flash drive. The process for each computer (or motherboard if you're building a gaming PC) will be different. When you first turn on your computer, there'll be a small line of text telling you which button to press to enter BIOS. If you don't see it, or it goes away too fast, I suggest consulting your manual for instructions.