Mac resources, service, and support
You can find more information about your iMac in System Report, in Apple Diagnostics, and through online resources.
System Report. To get information about your iMac, use System Report. It shows you your hardware model, the serial number and operating system version, how much memory is installed, and more. To open System Report, choose Apple menu 
Apple Diagnostics. You can use Apple Diagnostics to help determine if there’s a problem with one of the computer’s components, such as the memory or processor. Apple Diagnostics helps to identify the potential source of a hardware issue and provides first steps to try and resolve it. Apple Diagnostics also helps you get in touch with AppleCare Support if you need more help.
Before using Apple Diagnostics, disconnect all external devices except keyboard, mouse, display, Ethernet connection (if applicable), and connection to AC power. Be sure the computer is connected to the internet.
To start Apple Diagnostics, shut down your Mac, press and hold the power button until you see the startup options window, then press and hold Command-D on your keyboard to enter diagnostics mode. If prompted, select the language for your location. The basic Apple Diagnostics test takes a few minutes to complete. If an issue is found, a description of the issue appears with additional instructions. Make a note of any reference codes before you exit Apple Diagnostics, in case you need to contact AppleCare Support. See Use Apple Diagnostics to test your Mac.
Online resources. For online service and support information, go to Apple Support. You can learn about Apple products, view online manuals, check for software updates, connect with other Apple users, and get service, support, and professional advice from Apple. For more information about your iMac, go to iMac Support.
AppleCare support. If you need assistance, AppleCare representatives can help you with installing and opening apps, and with troubleshooting. Call the support center number nearest you (the first 90 days are complimentary). Have the purchase date and your iMac serial number ready when you call.
A complete list of support telephone numbers is available on the web at Contact Apple for support and service. Telephone numbers are subject to change, and local and national telephone rates may apply.
Your 90 days of complimentary telephone support begins on the date of purchase.