You can use the Display pane of Display preferences to specify how your display works. Not all options appear for all display models.
By default, the Display System preferences will automatically select the best resolution for your display.
To select a different resolution, use the Scaled option:
AirPlay Mirroring, if your Mac supports it, allows you to show your Mac's screen on your HDTV via an Apple TV (2nd or 3rd generation). This option doesn't appear on Macs that do not support AirPlay mirroring.
How can I detect displays?
If the detect displays option is not available, hold down the Option key while you are in the Display pane.
Choosing an additional resolutions
Hold the Option key and click on the "Scaled" choice to show additional resolutions.
For assistance with troubleshooting displays, please see Troubleshooting issues with video on internal or external displays.
For assistance with Retina displays, please see Frequently asked questions about using a Retina display.