It seems that your device is using Android Jelly-Bean OS by default. In this operating system, you can take a screenshot on your device's screen using a key combination. To take a screenshot, you need to press and hold down simultaneously the Power button and the Volume Down button. You should keep them pressed, until the screen flashes. This is the signal that the photo was taken. You can find the image on your Gallery.
On the other hand, you can use an application that was developed for this purpose only. You can search the Android Informer's database to find such an application.
