have you ever booted from a live usb?
the way to boot from a dvd varies by computer and can often be done a couple of different ways so pointing you to a video that shows you exactly how to do it on your computer is not an easy task. you might be able to find a video by searching “boot make and model from dvd” on youtube where you replace “make” and “model” with the information from your computer. mine for example would be “boot thinkpad w540 from dvd”.
the two different ways i have to boot from a dvd (or usb) would be to move the dvd to the top of the boot order in my bios/uefi settings or there is a slightly easier way that i get presented with when my computer starts up. i get a message that says something like “press f11 to boot from an alternate device”. as long as i press f11 quickly, i am then given the option to choose to boot from hard drive, usb or dvd.