If it doesn't have a CD drive you do it as Colleen said - from a USB drive. Usually a 4gig drive is big enough, and you can pick on up for as little as $10. You will usually need another computer with a CD drive to make the USB drive a bootable drive to install the OS.
Or if you do have a CD drive you pop the CD in, boot to it, and follow the instructions on screen.
If you are having troubles booting from the CD or USB drive there is usually an option when loading to press f12 for a boot menu, then select which device to boot from. Sometimes to boot from a USB drive its listed under the Hard drive or HDD group.
Do be aware that install a new OS usually ends up wiping the machine clean(you loose all data on it), unless you are specifically setup a dual boot.
Actually, do you mean notebook or netbook? Are you asking for specific instructions on how to reformat?.. Or?.. I'd be happy to help with that, if needed. Just FunMail me.
What Colleen said, or you can purchase an external optical drive and use a CD. (I bought one from WalMart for only $50, and it works great.)
You would have do download the OS onto a USB flashdrive.