It depends on time and how wealthy you were - we'll stick to the crusading period from ca.1190-ca.1290.

If you were poor, you walked the whole way, across France, through the Alps, down through the Balkans into the Byzantine Empire, across Asia Minor, and down into the Levant from the north, regardless of time period.

If you were rich and English, it's probable you would have sailed - more on that in a sec. If you were rich and not English, you either walked as well or possibly sailed from one of the Italian city states (probably Venice), the latter becoming more probable as time progressed.

The major shift in transport can be mainly seen in the crusades themselves:

The First Crusade (ca.1096-1098) went overland in two waves. The first wave, the "People's Crusade" led by Peter the Hermit was annihilated in Anatolia. The second, better armed and supplied, made it through and took Jerusalem. This crusade was mainly composed of people from modern-day France.

The Second Crusade (ca.1146-1149) was a single group, and followed the same route as the First, composed of people mostly from northern France and Germany. This crusade was also more or less annihilated in Anatolia. A small contingent of Crusaders from England and the Low Countries, however, did travel there by boat, stopping off to capture the port of Lisbon before arriving safely.

All crusades after the Second traveled by ship, having learned their lesson. Travel times could vary widely based on where you were coming from and when you got to port - you didn't want to sail in the winter. Roughly speaking, it took two months to cross France and another month or so to sail to the Holy Land. For walking, that could definitely take as long as a year, although three would probably mean you ran into some interesting adventures.