Mark 14:12 And the first day of unleavened bread, when they killed the passover, his disciples said unto him, Where wilt thou that we go and prepare that thou mayest eat the passover?
They asked him where the Passover meal would be. Judas had already betrayed Jesus to the chief priests, if they knew where the Passover was to be held, perhaps they would be waiting there for him to take him away?
Mark 14:13 And he sendeth forth two of his disciples, and saith unto them, Go ye into the city, and there shall meet you a man bearing a pitcher of water: follow him.
Only two of his trusted disciples were sent to find the room, and they were to look for a man bearing a water pitcher. The custom was that the women carried the pitchers, the men had flasks made of skins. When you see this man with the pitcher, follow him to the room.
Mark 14:14 And wheresoever he shall go in, say ye to the goodman of the house, The Master saith, Where is the guestchamber, where I shall eat the passover with my disciples?
Apparently Jesus had made arrangements with the man beforehand, in secret. Perhaps the man had come to Jesus earlier offering the room to him.
Mark 14:15 And he will show you a large upper room furnished and prepared: there make ready for us.
It was already prepared for them. I had attended a Passover meal, a Christian Passover with friends in the upper room of a restaurant. They had prepared it earlier, but over the head table was a sign advertising beer. They had overlooked this. We removed it before the service. The room that Jesus would partake of his last Passover supper, was all furnished and ready for them, with no image of beer. The word images would be of wine and his broken body for sins.
Mark 14:16 And his disciples went forth, and came into the city, and found as he had said unto them: and they made ready the passover.
They made ready the Passover meal. This would be an important event for all. At this event Jesus would announce the new covenant in his blood.