How to prepare my iMac to give it to someone?

Click here and follow the instructions. If the computer originally shipped with Mac OS X 10.6.8 or earlier, when you reach step 6, insert a compatible Mac OS X install DVD, restart with the C or Option key held down, use the Disk Utility to erase the internal drive, and install a fresh OS.

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Welcome to the Apple Support Communities.

Please see: What to do before you sell, give away, or trade in your Mac - Apple Support.

Jack

Please refer to What to do before you sell, give away, or trade in your Mac - Apple Support.

Follow all these steps:

https://support.apple.com/en-us/HT201065