To find out, what exact Mac model you use, click > About This Mac > More Info > Hardware Overview:
To see a more elaborate image explaining the above steps, click here.
To see a more detailed video tutorial showing the above steps, click here (2 MB, 23s, .mp4).
If you have Mac OS X 10.7 Lion or OS X 10.8 Mountain Lion, go to > About This Mac > More Info > System Report or Applications / Utilities / System Information.
We are interested in the MODEL IDENTIFIER. And also tell us what Mac OS X version you use.
I see from your previous posts you are on Mountain Lion on a 2011 iMac.
So just command-r boot to the recovery partition then start Disk Utility and erase Macintosh HD. Quit Disk Utility then click install OS. This will take some time as the entire 4.7GB OS needs to be downloaded from Apple's servers.
Make sure you have a backup before you do this as it will wipe all your data.
You can simply press the "Option" key when booting up, select the Recovery partition, and erase the drive using Disk Utility, and then you can install back on the erased volume.