If you want to make a screenshot on your mac you have to know the keyboard-shortcuts to do it (or have an app).
Command – Shift – 3 : Make a screenshot of the whole screen and save it as a file onto the desktop.
Command – Shift – 4 : With this shortcut you can select an area (with the left-mouse button). After the selection the screenshot will be also saved onto the desktop as an file (png)
If you want to save the screenshot into the clipboard instead of an file press also the Control-Key (Control – Command – Shift – 3 or Control – Command – Shift – 4).
Here is the link to the offical support-document.