If you make screenshots on your Mac often, you may want to launch and use the built-in app GRAB by going to Applications > Utilities > Grab.
However, if you only need to capture the occasional screen grab, you may find these shortcuts faster.
Command + Control + Shift + 3
This shortcut copies your entire screen to the clipboard.
Command + Shift + 4
This shortcut creates PNG file of the area you select from a crosshair click-and-drag selection.
Command + Shift + 4, followed by Spacebar
This shortcut changes your cursor into a camera that automatically captures the highlighted window, whether fully visible or not.