Well, click your mouse to leave the screen saver for starters.
If mirroring is turned on, you'll see what's on your Macbook's display. So if the screen saver is on, that's what you'll see.
Then turn mirroring off and you'll see a desktop with no Dock, no icons and no menu bar on your TV screen. Then you can drag windows and icons from your Macbook's screen onto the second desktop on your TV screen. Just drag a window towards the edge of your Macbook screen and keep dragging. It will slide off of that screen and you'll see it start to come in from the edge of your TV screen.
Think of it as an extension of your Macbook screen, as opposed to a separate screen with its own parameters. In other words, if you have both screens side by side, Macbook on the left and your TV on the right, both of them together make one big desktop. The left edge of your Macbook is the left edge of your desktop, the right edge of your TV is the right edge of your desktop. The right edge of the Macbook display and the left edge of the TV would be the "middle" of your desktop with a separation running down the middle. In fact, you can position a window so that half is on one screen and half is on the other.
To change which side your external display will appear on, go to System Preferences>Hardware>Displays.
