Transfer your data to your new iMac
It’s easy to move your files and settings from another Mac or PC to your iMac. You can transfer information to your iMac from an old computer—or from a Time Machine backup on a USB storage device—either wirelessly, or with an Ethernet cable and adapters.
Tip: For best results, make sure your iMac is running the latest version of macOS. Open System Preferences, then click Software Update to check for updates.
Transfer wirelessly. To transfer the data when you first set up your iMac, use Setup Assistant. To transfer data later, you can use Migration Assistant. Open a Finder window, go to Applications/Utilities, then double-click Migration Assistant to do a wireless migration. Follow the onscreen instructions. Make sure both computers are connected to the same network, and keep the computers near each other throughout the migration process.
Tip: For fastest migration, connect your old computer to your iMac with a cable, such as a Thunderbolt 3 (USB-C) cable.
If you used Time Machine to back up your files from another Mac to a storage device (such as an external disk), you can copy the files from the device to your iMac. See Back up and restore your Mac.
Copy files from a storage device. Connect the device to the USB 3 port 
or Thunderbolt 3 (USB-C) port 
on your iMac, then drag files from your storage device to your iMac.
Learn more. For complete details about all the migration methods, see the Apple Support article How to move your content to a new Mac.