Taking screenshot is very straight forward on MacOS. There are 3 ways for it. After you take screenshots, they are saved at your desktop.
To take screenshot of your whole screen simply press Command + Shift + 3 keys together.
Part of Screen
To take screenshot of specific part of your screen is similar jus press 4 instead of 3.
Command + Shift + 4. Then choose any part of screen with your mouse cursor.
To take screenshot of a single application just press Command + Shift + 4 at the same time. Then press space bar and then choose the application window you want to take screenshot.