The tensioner is the pully closest to the alternator. should be down and to the left
you need either a breaker bar (socket rench that doesnt rachet) or an N wrench depending on weather there is a bolt or square hole.
turn this CLOCKWISE as far as you can and HOLD IT until you (or someone else!) takes the belt off of the alternator. then GENTLY release the tensioner. take note of the layout of your belt. there should be a diagram somewhere under the hood that shows you.