When you switch the PC on, the first splash-screen you see will normally have an instruction on it like "Press "del" to enter setup", or something similar.
That's the Basic I/O System, which is burned into the motherboard chipset, and is responsible for getting your Windows (or other) System loaded into storage and running.
Follow the instruction (quickly, it's only there for a second or two), and you'll end up in the BIOS setup utility (which looks like an OS from the early 1980's).
Find the "boot" section, and set CD/DVD to be before HDD in the boot sequence.