You first have to select/highlight what you want to copy.  Do this by holding the left mouse button and dragging across what you want to select.  Then hover your mouse over the highlighted area, right click, and choose copy (or Control X).  Then move the mouse to where you want to past, right click, and choose paste (or Control V).  Sometimes (usually when working across different programs) paste will not be available with the mouse and the Control V option will work.