You need to boot into the BIOS (Sometimes called 'setup' or similar). It should tell you how when it boots normally. Usually involves pressing a key at a specific time during boot. Often the F1 key.
Once in BIOS, you need to tell the PC to boot from its optical drive. Under the ATA or IDE setup page, though precise nomenclature varies from machine to machine.
Follow the instructions after that to install a new OS.