To be thorough: there was more to the plan. Emperor Alexios I of Byzantium had requested help from the pope against invading Turks, and the pope saw an opportunity to help his Christian brother and then, since a western European army would be so close (800-900 miles!), why not take back the city that had been occupied by non-Christians since the 7th century? Expeditions like this required careful planning, and so the pope intended that it should begin in August of 1096.
Unfortunately, "crusading fever" spread quickly, and the spring of 1096 saw a movement of tens of thousands of peasants and lesser knights from across Western Europe amassing in separate groups and making their way toward the Holy Land. These various groups have been called the People's Crusade, the Peasants' Crusade, and the Paupers' Crusade. One group, led by a Walter Sans Avoir (Walter "Haves Not"), went through Germany and Hungary relatively peacefully, but reached the Belgrade area having exhausted their supplies. The leaders of Belgrade had no idea what to do with the newcomers and refused them aid, whereupon the "crusaders" took what they needed from he Belgrade area, causing much consternation and destruction.*
Other crusading groups (there were five major groups prior to the official and organized army) traveled down the Rhine and, finding communities of Jews, took it upon themselves to slaughter them or force them to convert to Christianity. Estimates of casualties among Jews range from 2,000 to 12,000.
The largest group was led by a priest from Amiens, Peter the Hermit (d.1115), who preached the Crusade in France. He rode a donkey and claimed to have a divine commission to lead. He arrived at Constantinople with 30,000 followers, putting Emperor Alexius I in the position of needing to provision this "army" (which included women and children). Walter's group and others showed up as well. Constantinople could not play host to so many needy tourists, and Alexios agreed to ship them across the Bosphorus to Turkey, telling them to wait while he arranged soldiers to get them through the Turkish territory. Crusading fever would not allow delay, however, and the largely non-military masses advanced, to be cut down in the thousands by the Turks. Wounded, starving, and penniless, the handful of survivors could only wait with Peter (Walter had been killed by several arrows at once) for the real army to arrive.
Jerusalem was captured by the armies of the First Crusade, but none of the success can be attributed to any of the tens of thousands of people who set out months early with little but faith on their side.
*I think of Dan Ackroyd and John Belushi in the movie "The Blues Brothers": "They can't stop us: we're on a mission from God!"