how to calibrate?

How can I calibrate my iMac?
I am new on using mac computer.
thank you.

I'm assuming you're talking about color calibration of the display. Open System Preferences->Displays->Color and click the Calibrate button. I use Expert mode. First adjustment, crank contrast all the way up, then adjust brightness so you can just see the oval in the box, but you can't tell the two halves of the box are different shades. Click Continue. From this point on, you're trying to make the Apple in the box match color and shade so that it disappears against the background square, or as close as possible. The left control adjusts the brightness of the Apple, and the right controls shading and hue. Work through those five or six adjustments. Set your screen temperature, set your white balance, and save. This process works for most people, but if you need a more professional calibration solution, you might want to use something like a X-Rite i1Display 2 Colorimeter / color calibrator