You need to configure this at boot-up, in the BIOS settings. As your computer starts booting, press F2 multiple times till the BIOS settings come up. On some computers you'll have to press F11 instead - normally your computer will tell you as it boots which key you need to use to get into the BIOS settings.
In the BIOS settings, there'll be a section to configure the boot-up sequence. This will normally be:
- HDD drive
- CD/DVD drive
- Network boot
It'll also have instructions on how to change the sequence so CD/DVD drive gets a higher priority. Do that so CD/DVD drive is at #1, which will automatically change HDD drive to #2. Save the settings using the instructions on the screen and exit. If you don't know how to save the settings, press Esc and you should get a prompt asking if you want to save and exit.
Let the computer reboot. It should detect your LiveCD and boot into that.
When you are done, you should change the boot-up sequence back.