Learn how to move your files to your new Mac. Do this before you erase the hard drive or follow any other steps.

Create a backup

Be sure you have an up-to-date backup of your important files and data. Learn how to back up your data in macOS. If you have macOS Catalina, skip the sign out of iTunes step.

Sign out of iTunes on macOS Mojave or earlier

Open iTunes. From the menu bar at the top of your computer screen or at the top of the iTunes window, choose Account > Authorizations > Deauthorize This Computer.

When prompted, enter your Apple ID and password. Then click Deauthorize.

Learn more about deauthorizing your computer using iTunes, including how to deauthorize all the computers you've used with your iTunes account.

Sign out of iCloud

Choose Apple menu  > System Preferences, then click Apple ID. Choose iCloud in the side panel and Turn off Find My Mac. Then select Overview in the side panel and click Sign Out.

If you have macOS Mojave or earlier, choose Apple menu  > System Preferences, click iCloud, and then Sign Out.

A dialog asks whether you want to keep a copy of your iCloud data on the Mac. Because you'll reformat the hard drive in a later step, just click Keep a Copy to proceed.

After you sign out of iCloud, your iCloud data remains in iCloud and on any other devices you've signed in to with your Apple ID.
   

Sign out of iMessage

If you're using OS X Mountain Lion or later, sign out of iMessage.

In the Messages app, choose Messages > Preferences, click iMessage, then click Sign Out.
  

If you're keeping paired Bluetooth devices, unpair them (optional)

If you have Bluetooth devices — such as keyboards, mice, or trackpads — paired with your Mac, and you plan to keep these devices, you can unpair them. This optional step prevents accidental input on the Mac if the computer and the Bluetooth devices have separate owners but remain within Bluetooth range of one another.

If you're unpairing an iMac, Mac mini, or Mac Pro, you must have a USB or other wired keyboard and mouse to complete these steps.

To unpair your Bluetooth devices, choose Apple menu > System Preferences, then click Bluetooth. Hover the pointer over the device that you want to unpair, then click the remove (x) button next to the device's name. When the dialog asks if you're sure, click Remove.

If you're using an iMac, Mac Pro, or Mac mini, use a USB or other wired keyboard and mouse to complete the next step.
   

Erase your hard drive and reinstall macOS

The best way to restore your Mac to factory settings is to erase your hard drive and reinstall macOS. 

After macOS installation is complete, the Mac restarts to a Welcome screen and asks you to choose a country or region. If you want to leave the Mac in an out-of-box state, don't continue with the setup of your system. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, setup assistant will guide them through the setup process.

@Credits: Apple