In About This Mac
From the Apple menu in the corner of your screen, choose About This Mac. You should see an overview of your Mac, including its model name and serial number.
If you see a window like this, double-click the version number beneath “OS X” to show the serial number:
In System Information
The System Information app also shows your Mac model and serial number. Open the app, then select Hardware on the left side of the System Information window. The model name and serial number appear on the right:
On the product or its packaging
Sign in to appleid.apple.com, then look for your Mac in the Devices section. If you see your Mac there, click its name to see the model name and serial number.
If you still need help, contact an Apple Store or Apple Authorized Service Provider.