How do I wipe out an iMac (aka Fickle, fickle Sue)

I can’t do it. I know I said last night that I was going to keep it but I called to ask if I could return it for a new Vaio and they said “no problem!”

So now my problem is how do I remove all my info and preferences from it? It’s running OS X and I know nothing about macs.

Much obliged,
Sue