How do I reset/clean up my iMac before giving it away?

I got a new IMac and want to give my old one to my son. In particular I want to be sure that when I delete my contacts that they will still be on my new Mac and iPhone and iPad. So do I do the following?

Sign out of Find My

Sign out of my Apple ID account

Delete all contacts

I assume the same thing for a mail? Am I forgetting anything?

[Re-Titled by Moderator]

Original Title: How to clem up iMac to give to someone