The answer is very simple if you are using Android version 4.0 or above.
All you have to do, is press Volume Down and Power buttons at the same time and hold them for a while (maybe second).
|Volume Down + Power buttons on Nexus 5 (source: Google.com)|
You will hear an odd sound and your Android will take a screenshot. The "Saving screenshot" notification will appear.
The file will be in folder SDCard/Pictures/Screenshots. If you don't have a File browser, you can also find it if you pull down the drawer with notification.
When you open the screenshot on your Android device, you will see the Share button and then everything is only a step away.
Simple, isn't it!