how do i clear my old mac of all info

Erase and install OS X from the Recovery partition.

Instructions here if the Mac came with 2 grey disks: http://support.apple.com/kb/HT3910?viewlocale=en_US

If you're passing the Mac on the disks should be handed over with it.

(Obviously everyting will be wiped, so copy everything you need somewhere else first 😉).

Boot ro recovery by holding COMMAND+R on boot up, until you see an apple and spinning gear. This will take you to OS X Utilities. Once in OS X utilities, select Disk Utility.

Once in Disk Utility, select the Mac HD, and go to the erase tab. Make sure the name of the HD stays the same and the format is Mac OS Extended (Journaled) then click erase.

After you do this, exit out of the disk utility, go back to OS X utilities and click reinstall OS X.

Easy peasy. 😉

And, an additional note: if you are selling or giving it away, you need to erase the drive and reinstall the original OS on it; if you obtained Mountain Lion (or any other OS or app) at the app store, you have to remove it because it is not transferable and is tied to your Apple ID forever.