Press and hold Power and Volume down key simultaneously.
Continuously for three seconds.
When you hear camera shutter sound. You are done.
Please note that both the key must be pressed simultaneously till you hear camera shutter sound. If you press power key early you will have power options on your screen. In the same way if you hold volume key early you will have volume option on you screen. You can see your successfully taken screen shot in internal memory of your phone.
There are also some apps available to take screenshots.