How to nuke everything from imac and start it at the beginning from scratch like i first bought it
The easiest way is as follows:

You will need the original (or a newer) Mac OS X install disk for the computer. Do not use and older version than originally came with the Mac, and do not use the install disk from any other model of Mac.

Insert the disk and restart the computer pressing the Option key as it restarts. After about a minute when it loads, click the arrow under your installer DVD to boot from it.

When it starts, click continue on English. In the menu bar, select Utilities and select Disk Utility. Disk Utility may be under a different menu on some install disks.

Select your disk in the left column (select the disk, not the partition (select the one labeled something weird, not "Macintosh HD")) and follow the instructions to erase the disk. For best results choose the option to "securely erase" the disk, or to overwrite the data with 0's rather than just mark as free space. It will take longer but is more secure (if your data was valuable) and cleans the disk better. Then close Disk Utility and install the operating system on the newly erased disk.

Viola! Brand new Mac OS X. Enjoy the startup video ;)