The recycler for the E-waste event next week guarantees that the information on your hard drive will be securely deleted when you discard your old Mac, but if you want to erase the hard drive and delete all the files for the best security before you drop it off, you can.
What you need to do is start your iMac up from the Apple software DVD that came with it. Put the OS X install DVD in the drive and then restart, and be ready to hold down the “C” key right after you head the startup sound. Holding down that key will make the Mac start up from the DVD rather than the hard drive.
It will take a minute or two, but when you get to the installer, Select English for your language. When that finishes starting up, select Disk Utility from the Utilities menu. Disk Utility will start up in a few moments, and from there, you can erase the hard drive.
Select your hard drive in the left hand pane and then click the “Erase” tab. This will erase your disk OK, but if you go to the “Security Options…” button near the bottom right corner (it’s right next to the “Erase” button), you will see more options. You can select “Zero Out Data,” which is a more secure method of erasing files and should be fine for your needs. There are two other options available, as well: 7-Pass erase and 35-Pass Erase, which are more secure (read the description) but take more time.
Once the disk is erased, shut the iMac down (don’t install the system software), and the iMac is ready to go to the recycler with all the software, files and information erased and deleted to a degree that it won’t be easy for someone to recover them.