first fdisk -l and get the drive divided up with your partitions: sudo fdisk /dev/hda
then format partition 1 for etxt3 as an example: sudo mke2fs -j /dev/hda1 and so on for next partitions.
I want to reinstall on a different First HDD, but the disk needs to be reformatted.
I would like to install an old Win98SE first, Then SuSE Linux again, but how do I first reformat the disc please?
The first (/dev/hda1) will probably want to be vfat (fat32) if Win 98SE is going to go onto it.
That said, if this is the only hard-disk in the box then a Knoppix or other live-cd distro would allow you to format the disk while running from the cd-rom drive.
Cfdisk, fdisk GnuParted and Gparted are all tools you may find on live-cd.