I am giving a 2013 Mac Mini (server) to a local non-profit and was wondering what I need to do to securely delete personal information (passwords, iTunes store, etc.)?
Boot to the recovery partition (hold down Cmd-R while booting) Go to the Disk Utility and erase the disk. Then have it reinstall OS X. This will take a while, but when done the system will be in its pristine original state. Contact Apple about the change in ownership so the non-profit will be able to set up with the the Apple Store to be able to download the OS, iLife apps, and OS X Server in the future.
If you didn't encrypt the whole disk, then I would opt to do a simple zero out in Disk Utility -- it simply overwrites all the data with zeros -- and then reinstall the system. Don't do a 3 or 7 pass secure erase because it takes too long -- and also causes the Mac to heat up quite a lot.
How to authorize or deauthorize your computer with your Apple ID (you'll need to enter your password): Image
iTunes in the Cloud:
Go to menu->iTunes Store->View Account (you'll need to enter your password) Image
Click on Manage Devices: Image
Choose which device you want to remove Image