If you click your "fn" key (Bottom left if on a notebook, under home key if on a desktop), then you can use the default "F" keys. To be a little more mac specific you can take a screenshot anytime with the following commands.
To save to desktop:
Command-Shift-3: Screenshot of full screen.
Command-Shift-4, then drag and drop mouse over selected area: Screenshot of selected area.
Command-Shift-4, then space, then click a window: Screenshot of selected window.
Save Screenshot to Clipboard
Command-Control-Shift-3: Screenshot of full screen in clipboard
Command-Control-Shift-4, then drag and drop mouse over selected area: Screenshot of area in keyboard.
Command-Control-Shift-4, then space, then click a window: Screenshot of selected window in clipboard.
-Oprah