The easy way to take a screenshot on an Android device is to simultaneously hold down the volume down and power keys for a couple of seconds. You'll see a screenshot appear then vanish, it will be stored in your screenshots folder of Gallery.
Handy hint: sometimes it's tricky to hold down the keys properly when the phone is inside a separate case or shell. I found this with my Moto phone, so I slip the outer case off before doing the screenshots. It saves my temper and my nails!
