OK so I made a bootable USB with Kali on it and I've used it on two different laptops and it booted just fine, but when I try booting it on my laptop it just skips the USB and boots from the hard drive. The two laptops that it did boot on were running Windows (one had vista the other had 10) but on mine I have Ubuntu installed and before that I was running mint, I don't know if that's the problem but it's the only thing I can think of right now.
You need to go to BIOS at the machine startup and change the boot order. USB needs to be before HDD. You can probably go to BIOS pressing
F12, or some other
F. Look for an instruction just after you power up. There may also be a special boot menu allowing you to set the order for just one startup.