as jc98 stated: FN+Fkey
you will have to look at your keyboard to see which one.
you might have to push the combination a few times as it swaps between laptop lcd, laptop lcd + external and external only. this is also the combination used to connect projectors i believe.
when i output in this manner to the television it automatically matches my televisions resolution (1080p) but this might not be true for all televisions. sometimes when i travel the image overhangs the edge of the television and i am not quite sure why. on the two televisions at home there are no such issues.
check and see i suppose.