Boot from the suitable bootable CD-ROM. This might be one of the network installer single CD-ROMs from Progeny or Debian, or disk 1 of the Debian CD-ROM set (e.g., it might be called the Debian GNU/Linux Binary1 (nonUS) CD-ROM) or even the Rescue floppy disk. For different hardware boot from the appropriate media--for example for a SPARCStation boot from the Sparc CD-ROM.