Easy way, but not smart if that is the OEM edition of Windows 8.1 on it. The OS is ONLY allowed on THAT laptop, you can't use it on a 'new' computer. If you heed this advise then start in safe / recovery mode and you can restore to FACTORY which will wipe out the user data and put it just like you pulled it out of the box to begin with.
Otherwise you have to use a tool like DBAN to wipe it, but honestly no one wants a 'old laptop' with no copy of Windows on it (your price PLUS another $179 for Windows just to make it work? Screw that old laptop, I get a brand new one for only $200 at Walmart WITH warranties and BRAND NEW!) is what people think.