To take a screenshot, hold down the Command and Shift keys and press the number 4. Then use the mouse to select the area of the screen that you want to take a picture of. This will create a file called Screen Shot <date> at <time>.png on your desktop. Pressing the keys again will create Screen Shot <date> at <new time>.png, etc.
You can also take screenshots using the File ‣ Take Screenshot command in the Preview application.
To send in screeenshots, drag the files into your e-mail window to attach them to your message. If you’re using Apple Mail, be sure to set the Image Size pop-up menu at the bottom of the message window to Actual Size.