To boot from a USB drive:
Go through these steps:
1:Restart your computer and wait for that first screen to pop up. Often, it'll say something like "Press F12 or Delete key or F2 key(mostly in asus laptop its delete key) After Pressing the key ,Boot Menu will come up.
2: Go to boot options by navigating your arrow keys on your keyboard
3: Select Boot priority by hitting enter key and select the first boot priority to Usb device and second priority to Hard disk.
4: save using F10 button and system will restart automatically and it will ask press any key to boot from usb .Just press any key to boot. Thats it.. Done.