Hide in the bushes and sneak around to the door with the two people standing there.
"You're now required to do a tutorial/training course. The training course will require you to follow the green points, You need to run towards them whilst holding down the free run button. That way Desmond will automatically pass all the obstacles in his path.
"(Optional Objective) Perform an Air Assassination
To air assassinate head to the end of the beam and the enemie below will become highlighted, when this happens press the attack button and you will assassinate the enemy below. Kill the next enemie in the same way using the beam above him.
"Continue onward, hold the free run button and Desmond will automatically pass all the obstacles. Desmond may hesitate to jump if this is the case continue to hold the free-run button and press the jump button in order to make Desmond jump forward.
"Continue forward until you reach the end of the obstacles.
"Once you've reached the end of the obstacles you will change into Haytham. Walk toward the door at the end of the path. Once you've reached the door interact with it to begin Haythams first mission.