Depending on if it is a laptop or desktop..
Desktop...
Enter the bios during boot sequence (DEL/F1 ...or some other magical key sequence depending on the manufacturer)..
Find where the boot sequence is and set the CDrom as the first boot device or Floppy/CD/HD..
Save your settings..
Insert win 98 CD..
Reboot ... System will boot from CD and ask you if you want to run setup..
Laptop ...Similar method but the key entry to boot from CD varies even more. do a google on your particular make
If your laptop doesn't have a built in CD then it gets MUCH harder..I ended up removing hard drive and connecting it to a desktop via 2.5 to IDE adaptor ..copied win files across then reinstalled rebooted.
Good luck
MAx