Press command(⌘) + Shift + 3 keys simultaneously to take a screenshot of the whole screen.
Press command(⌘) + Shift + 4, and then drag the pointer to select the area for a screenshot.
The screenshots will be saved on your Desktop.
Please note that there are different Mac keyboards, so the buttons may differ. Sometimes Shift may look like an up arrow, and the command key might be named as cmd instead.