My motherboard in the laptop is broken, but I need to wipe out the hard drive before I sell it. How do I do th?

If the laptop's HDD is SATA (requiring a thin 5pin connector for data) then you can plug it into any desktop and wipe it. If it's an older IDE drive (using a 44pin connector) then you'll have to buy an adapter to put it in a desktop or you'll have to put it in another laptop with an IDE connector.