Press "Command+Shift+3" to capture the whole screen. Press "Command+Shift+4" and drag your mouse to capture a specific region. Press "Command+Shift+4" and then "Spacebar" to snapshot an active window. You can find the images saved on the desktop in PNG format.
You can use key combinations to take screenshots on Mac.
cmd+shift+3 is for a full screen
cmd+shift+4 is for a screenshot of a certain selection which you choose by using the trackpad