Android 4.0 and higher versions should have built in Screenshot capability.
Just hold Volume down and power button simultaneously for about 2 seconds. You will hear a capture sound and a Toast message. To get the captured image go to /sdcard/Pictures/Screenshots. In some mobiles Screenshots may be in Gallery folder. In some mobiles the above combination may not work. Then try the combination below.
Power and Home button.
If the above two combinations fail then you can take screenshot by using Screenshot Ultimate app. After installation this app will check whether your device has inbuilt screenshot capability.
This app is free. No root required. This app supports over 10 different capture methods. You can download this app from Google Play Store.