Insert the CD in the tray.
Tap whatever key accesses your BIOS right when the machine first gets turned on and before it boots into the OS. You may have to be quick. Sometimes it's F2, sometimes it's ESC, sometimes F1, etc. It's just a key press, no command. The key to press should flash on your screen right before it boots into the OS or it should be in the manual (BTW, what's the make and model number of the machine?)
Once you get into the BIOS, there should be a screen/section titled something like "Boot Order". In that screen you should be able to set the CD as first in the boot order . . . that way the machine will look to the CD for boot instructions BEFORE it looks to the HDD for them. That's what you want. When you set the boot order to the CD first, then press OK, or "Save and Exit", or "Apply" or whatever your BIOS requires to accept the setting.
Most of the time, the machine will then go ahead and continue the boot process, now looking to the CD FIRST for boot instructions, but if it shuts down, just go ahead and turn it on again.
I'm giving instructions in the dark here because we don't know the make and model of the machine. If you tell us the make and model (like "Dell Inspiron 1545" or something), we may be able to tell you what key to press to get into the BIOS and what BIOS screen to look for.