Aside from the shortcut key strokes you can use to make screen capture: (Command-Shift-3 and Command-Shift-4) you Mac also has a neat built in app called Grab. Here is how you use it.
Go up to your spotlight (magnifying glass at upper right) and type in Grab. The top hit you get should be the app. When you click on it the app will show up on your bottom tray.
Select the app and then go up to the top menu and click Capture. Here you can specify whether you want to capture a selection, the screen, the window or even a timed selection.