or, to make things nice and easy for you, when starting it up, press and hold command - r till the apple logo appears, then use disk utility to wipe the hard drive and remove any partitions, then install Mac OS X 🙂 that should get you where you want to be 🙂
i should mention that you should make sure you have a bootable install disk or drive before doing this, so that you can actually install an OS 🙂