The usual way to start using the internet or other applications on your computer is to click on the representational image known as an icon. Icons appear on "the desktop" -- the area that you see on the screen when the computer is on, but when no applications are currently open.
To open an application, place the pointer over the icon and click the left mouse button twice in rapid succession. This is referred to as "double clicking".
Remember, do not move the mouse between clicks-- the application will not open if the mouse moves.
Double-click quickly -- about as fast as you can say "clickclick".
Practice double-clicking the Firefox icon below. This will again cause a pop-up window to appear on the screen. Follow the directions in the pop-up window.