Go to the File menu, slide down to “Grab,” then choose either:
Selection:
Preview provides a crosshair icon. Press-and-drag around an area to take a screenshot of just that area.
Window:
Preview provides a camera icon, shown below. As you move the camera icon on top of different windows on your screen, it highlights each one. When the window you want is highlighted, single-click.
Timed Screen:
Preview gives you ten seconds to get your screen set up the way you want it for a screenshot. For instance, you might want a picture of a particular menu and submenu. After ten seconds, Preview snaps the screenshot of the entire screen.