All you really need is a bootdisk along with your win98 cdrom. Make sure you do have all the drivers you need, video, printers, audio, enet or usb modem etc.. before you do so as once you wipe the drive, they are gone.
Also make sure to back up your data to cd as well; documents, pictures, emails, email address book, favorites that sort of thing.
Go here to get a win 98 bootdisk, make sure its the one you need as win98 has a few flavors : http://www.bootdisk.com/bootdisk.htm
After backing up everything, toss in the floppy disk and reboot your machine. You may have to go into BIOS to set your pc to boot from the A: first. Do that, reboot and it will run the bootdisk.
Simply type format c: at the prompt if you have just the one drive/partition to clear. Or if you have multiple partitions and want to resize or make any adjustments you will need to type fdisk at the prompt to get those options.
Once you clear the drive, remove the floppy, insert the win98 cdrom and reboot. Go back to BIOS and set it to boot from cdrom and restart again. That will take you to the win98 setup from cdrom and just follow along with the prompts. Good luck.