On a Mac
Press ⌘ Command + Shift + 3 to capture the entire screen, or, to select an area of your screen to capture, press Command + Shift + 4. Your cursor will turn into a crosshair and you can drag it around the area you want to capture.
On a Windows Computer
First check to see if your keyboard has a Print Screen (sometimes "PrtScn") key. Pressing this key will capture an image of the whole screen, which will be placed in your clipboard. You can paste (Control [Ctrl] + V) the image to an email or message.
If you don't have a Print Screen key, you can go to Start→All Programs→Accessories and choose the Snipping Tool. You'll be able to use your mouse to select the area of your screen you want to capture.
On an iPhone
Press and hold the Power button and click the Home button. The image will be stored in your Photos app.
On an Android phone
If you're running Ice Cream Sandwich (4.0) or above, press Volume Down and Power at the same time. The image will be stored with your photos.
Phones with Android 2.3 or below don't always have screenshot capabilities, but on most Samsung phones, you can press Home and Power at the same time to take a screenshot.