That's easy. You get an OS install disk, boot from it and install the OS.
The ones I see on eBay have 256 or 512 MB RAM. That limits you to using Windows XP or a small version of Linux. But I'm afraid that with Windows XP you'll have trouble to find the necessary drivers to fully enjoy all this laptop had to offer.
I recently received an old Dell Latitude C840. It had it's hard drive wiped with a think vantage program. It is able to boot but ends up at a black screen with text.
How can I install an operating system?