older macbook

Pop the disc in, boot holding down Option, choose to boot from the disc. When it's ready, select Disk Utility, click on the hard drive and select 'format' in Disk Utility. Quit, and then select Install Mac OS X from the menu.

 

Last white MacBook. One easy way if you want the current OS is to make a USB installer with DiskMaker X. Just re-download the latest OS X from the App Store and run DiskMaker X with an 8GB+ flash drive. Then hold the option key while booting and select your USB drive. You can then use Disk Utility to erase the drive and reinstall OS X fresh. Just like new.

Or if you want the original OS. Go with the DVD's as stated. Probably some version of Snow Leopard I'd imagine. It'll get to 10.6.8 and you can use the other disk to reinstall things like iLife and iWork if you have them.