Don't listen to the previous post unless you are using a pc.
Since you say you are on a mac do one of the following:
"Apple" key + "shift" key + "3" key will capture the entire screen.
"Apple" key + "shift" key + "4" key will enable you to drag the cursor over the area you want to capture. Release the mouse and voila.
Both will place a .png file on your desktop. Example: "Picture 1.png".