It's quite unlikely that F8 is the keypress to access BIOS setup. That's the keypress usually reserved for accessing Windows 'Safe Mode'. Most motherboards require the key to be pressed while the system is first powering up in order to access BIOS setup, although proprietary 'name brand' PCs may have a different keypress.
See this article for further information.
Once in BIOS setup look through the menus to find the items which related to 'Boot order' or 'Boot device'. The keys used to navigate the menus and make changes will be shown onscreen, so there's no need to describe them.
You will usually have a list of entries on an 'Advanced' menu which will indicate the order of boot devices, and the most useful method is to set CD-ROM as '1st boot device' and HDD-0 as '2nd boot device'. Some older systems may have a single setting for 'Boot Order' and you need to cycle through the available options to locate the most relevent one. Ensure you Saves the changes before exiting BIOS setup.
Some name brand systems will boot directly from CD without an onscreen message, but most assembled PCs will display an onscreen message asking you to "Press a key" to boot from CD, so you need to be watching the screen during the power-up process or you'll miss it, and the system will try to boot from hard drive or elsewhere regardless.