Most newer (less than a few years old) laptops -
Plug in your external monitor, and just close the laptop. Unless you have your laptop set to Standby or Hibernate when you close the screen, the laptop will automatically switch to the external screen.
Otherwise, hold the Fn (known as the 'Function') key, and press (usually) F8 - you should see some blue lettering saying LCD/CRT on the F8 key. If not, hold down Fn while pressing whichever key DOES say LCD/CRT on it.
On a side note, if you need the LCD fixed, PM me - I do a wide variety of computer repairs. SM members get a discount on any computer work I do for them