Any of the proper Linux texts will tell you about this. I think it's worth spending $50 or so on a decent book on Linux, because this will help you at the disillusioning stage, i.e. getting started.
Fundamentally though, what you need to do is partition the hard disk into a FAT32 partition for Win98, and then unpartitioned space, using FDisk. Then install 98SE as normal.
Next install Linux, allowing it to partition the remaining space on the drive. Now this is the important part: you must not overwrite the Master Boot Record (MBR) with the Linux installer, because this will break Win98SE, and you'll have to start the whole shebang again.