There are many applications for taking screenshots on your phone but since Android has a built-in screen shout feature you won't need them.
- At the same time press and hold power and home button on your phone
- Wait for a second or two until you receive notification that your screenshot is successfully taken
(depending on your phone settings it could be a screen flash or blink, vibration or sound)
You will need some practice at first a few attempts but soon it will become a routine operation for you.
After taking a screenshot you can find your picture in your notifications or later in your Phone Gallery.