Applecare Certificate

Did you purchase it directly from Apple online? If so, you should received an email at approximately the same time as you receive your new Mac and you can print that email the same way you print any other email. The exact details depend on what email app you are using.

Also, while having proof is a good thing, I have never needed any proof for AppleCare. It is linked to the serial number of the device and you can verify it at https://checkcoverage.apple.com/

The receipt should show that the person purchased Apple Care. When I purchased online from Apple, I got the email mentioned in my original reply. When I purchased another product from an Apple Authorized Reseller, all I go was the receipt showing AppleCare. I then verified I had the coverage by going to https://checkcoverage.apple.com/

If someone else bought it and without purchasing a computer at the same time, you will need to register it so it covers your device. It will not be activated unless it is registered with a device.