It will be easy If you are using Android 4.0 or later
Just press and hold the Volume Down and Power buttons at the same time
You will see an animation on the screen
indicating that the screenshot was saved
Android will save the screenshot to your Gallery
Launch the Gallery app and tap Screenshots to view and share your screenshots.
If you are using Android 2.3 or later, then follow this trick
Press and hold the home key for 1secs and immediately press the power button after a vibrate release both button,then your current page will be snapped.
You can find your screenshot in sdcard/ ScreenCapture.
Note: You have to press the home button for almost 1sec (before task manager comes up) then press the power button almost simultaneously and after a vibrate release the butons immediately.
This trick works on almost all Android devices,but if it doesn’t work try the alternative method.