In true Assassin’s Creed style the first chapter is always the tutorial, with you playing a Templar like in Assassin’s Creed IV. After a scene you will need to Follow de Molay, so press on the Left Stick to walk around and hold R2 / RT to sprint.
After only a short walk you’ll get another scene and be told to hide a sword and book from the Assassins. Sprint towards the green waymarker in the distance and you may find yourself getting pulled into combat. These enemies can be ignore by sprinting away or attacking them with Square / X to kill them.
As you make your way into the courtyard another scene will occur with the drawbridge being raised and the camera will spin toward you next destination. Run up the steps and climb the ladder up to cut through a room to the scaffolding on the other side and then climb up until the scaffolding ends. You can climb the side of the building here by holding R2 + X / RT + A to jump up and then using the Left Stick to maneuver up the wall.
Enter the building to get Smoke Bombed by an Assassin and the turn right and head downstairs for a scene to find that the Assassin has managed to steal the book. Rush out the window, making sure to hold down R2 + X / RT + A, to hop across the gap and pull yourself up and then run after the Assassin. When the Assassin runs down the steps to the left you should continue straight, because you’ll lose him either way, and then jump down when the waypoint appears for a scene.
The battle that follows acts as your first official tutorial for combat. You will want to press O / B to parry the Assassin’s attacks and then attack with Square / X. The trick is to watch the diamonds above the Assassin’s head and when a bar appears it means he is going to attack. You want to try and parry when the bar flashes yellow to perform a Perfect Parry, which will stun the Assassin and allow you to get a chain off.
After a bit the Assassin will drop another bomb in an attempt to escape, but will be stopped when he has to deal with another Templar. This section will teach you to Dodge using the same timing as a Parry, but instead of pressing O / B you want to press X / A. In general this is less useful, but when dealing with Heavy Attacks from Brutes that can’t be parried you will want to use this.
Once the Assassin is dead you will trigger a long cutscene, beginning the true story of Assassin’s Creed: Unity.