If you don't have a specialist piece of software for taking screenshots you can use the standard functionality available in your operating system.
On a Mac the best way to take a screen shot is to hold down command-shift-4. This will bring up a cross-hair icon, you can use the mouse to drag over the area you wish to screenshot and let go. You will hear a camera shutter noise and a screenshot will be saved as a timestamped PNG file on the desktop.
Alternatively instead of dragging the mouse click the space bar, a camera icon will appear and you can click on the window you wish to capture.
For windows focus the window you wish to screenshot and click Alt+PrtScn. This will copy the screenshot to your clipboard and can be pasted into your email program.
Another way to capture a picture of part of the screen is to use the Windows Snipping Tool. To learn more, see Use Snipping Tool to capture screen shots.