I think you need an 8 unit army to join a crusade, then just doubleclick the general or merc hiring button to bring up the info window, and there will be a button in the bottom left. Once you join the crusade you get access to cheap "Crusaders" like Pilgrims, Religious Fanatics, Crusader Sergeants/Knights/etc... Its pretty easy to build an army of these cheap mercs on the way to the holy land. For reward you get experience given to all units in the crusading army, and like 1-2k florins usually as well as a boost in Papal relations. Of course, if its your army that conquers the target you can also sack the city. Most of the holy land cities will net you 10k+ on a sack, so a crusade can be a profitable venture.

One other thing to note is that during the crusade, you don't have to pay any of the crusading armies units upkeep costs. So with some good management, crusades are a boon to your economy.