Before you get into the game, you have to pass this simple test. Depending on which controller you prefer, there will not be any button references throughout the Campaign.
First, pick up the standard pistol that you'll be using for the whole single player experience. Go to the middle area for some target training. Shoot five targets by aiming for the red dummies. Then use your aim sight to do the same thing. Crouch and aim sight to finish the final part of it. Lastly, don't forget to reload. Pick up the remaining weapons and try them out.
It's worth noting you'll be permanently using you standard pistol throughout the game. Your two other weapons can be swapped anytime. Most of all, you can't use grenades whatsoever. Since you're a spy and not a soldier, guns and unarmed combat is all you can rely on. Lastly if you're standard pistol runs out of ammo, you're going to have to use other weapons that you picked up or choose to make up for that loss so be wise with your gun choices. Since there's no ammo anywhere on any levels for your standard pistol, you have to be smart about conserving ammunitions when it comes to heavy gun battles or stealth missions that requires long distance strategy if you can't get up close without being noticed. Make sure when you pick up weapons on the ground to frequently reload all weapons so you won't have to worry about changing to a gun with a near empty clip for the sake of convenience.
When you're done, go to the door, hit the button and open the door. Turn right, move forward and press the button to leap over an object. Turn left, sprint straight and turn right. Just as you go down the stairs, put on the silencer and go over the last object.
Crouch down and open the vent. Go through and open the vent from the other side. Stay crouched and walk up to the soldier. When you're near the soldier, when you see a button or motion prompt, use it to knock the soldier out. Go left, jump over plants if you want to and take out the soldier with your pistol. You have a few seconds before he calls backup. You'll start again if you fail within an alerting moment so keep shooting until the persons knocked over. To exit this tutorial, go through the elevator to start on your first mission.