Well, this would require knowledge of DOS commands. First of all, do you have the win 98 boot floppy? If not, go
here to download a version of FDISK. What FDISK does, is allows you to manage partitions on your harddrive. The most common use now is to format a drive (hence the expression, fdisk your harddrive).
To be honest, I haven't worked with 98 in a while. Try popping in the CD when you boot up and see if the CD can do all of this for you. It'll probably be a little less stressful. If it can't, you'll have to run FDISK to reformat your drive, then install 98.