I am going to give my I-MAC to a friend and I want
to clear all of my personal data from the computer.
What is a recommend method. Thanks.

The safest way is to reboot to Recovery, erase/repartition the drive, and reinstall macOS. This can be done over the network, or from a thumb drive loaded with an appropriate version of macOS. The exact details and keystrokes depend on the OS version you are currently running, and the age/architecture of your hardware. Details at support.apple.com. Apple gives you the choice of reinstalling the OS the machine was shipped with, the most recent version that supports your system, or any release in between the two. You could also install Windows, Linux, or BSD, and really confuse your friend.

Of course, this will wipe out any third-party applications and all user accounts, but leaves the friend with a "fresh" machine.

An alternative that risks leaving some user data behind is to:

1) Create a new Admin user,
2) logout and back in as the newly-created user,
3) in System Preferences/System Settings remove the old user account,
4) in Terminal double check that the /Users/username directory is gone.
5) Run Keychain Access and verify that the old account's secrets are gone. You can also remove unneeded WiFi pass codes, etc.

Architect is right. Do the simple steps outlined. Leave about two hours of your time to babysit the process.