How can I turn off the screen on the macbook when I have an external monitor set up?
Turn on your MacBook without the external display connected.

Close the lid and it will go to sleep.

Connect your external monitor and an external keyboard via USB.

Now wake it up by typing on the external keyboard. If you don't have a USB keyboard (you might be using a BT keyboard and mouse) you can use the Apple Remote to wake up a MacBook when its lid is closed.

The MacBook should wake up with the external monitor as its one and only monitor.