Take a screenshot of the entire screen:
Press Command+Shift+3. This will place an image file on your desktop. Double-click on the file to open it in Preview.
Take a screenshot of just a portion of the screen:
1) Press Command+Shift+4. When the cursor changes to a cross-hairs, click and drag over the area that you want.
2) An image file will be placed on your desktop.
3) Double-click on the file to open it in Preview.