Go connect external display to your iMac, at > Connect an external display to your iMac - Apple Support
Look down at the bottom of that Apple Support article for the Adjust, Arrange and Troubleshooting links.
Adjust and arrange displays. After you connect an external display or projector, use Displays settings in System Settings to set how the displays are arranged, turn screen mirroring on or off, and make other adjustments to color, resolution, and more. See Connect a display to your Mac in the macOS User Guide.
Troubleshooting. If you’re not sure how many external displays your iMac supports, check your Technical Specifications. Open System Settings, choose Help > iMac Specifications, then locate Video Support (you may need to scroll). To troubleshoot an issue with an external display, see Get help with video issues on external displays connected to your Mac.