I own a ford focus 2006 the belt fell off I'm not sure which one and a need to be able to put it back on.

Well, the belt does not just fall off. If the belt has come off it's because one of the pullys have failed.there is only 1 belt on that car, there should be a diagram under the hood that says "drive belt routing"
You need to release tension on the tensioner pully with a serpentine belt tool or a long 3/8" breaker bar. rout the belt according to the diagram, then re apply the tension