The starting point in the first mission is at a distance from the target. First, you must get to the battlefield. Enter the car and your allies will do the same. The path to the target is displayed in the bottom right corner, on the minimap.
Leave the vehicle when a yellow circle appears on the minimap, indicating target's destination. Crouch, get closer to the first enemy and kill him stealthy. In the later part of the mission, where you have to face multiple enemies at the same time, use a synchronized shot. The respective button will appear in the bottom right corner, above the minimap. Use it to tag an enemy, a number "1" will appear above his head. When you press and hold the same button, one of your allies will shoot the tagged target. At the same time, you must eliminate the other one.
This is how you must get to the Santa Blanca lieutenant, who must be interrogated. You can find him near the tent. Remember that if you kill him, you will fail the mission. You must approach him and grab by using the action button. After that, the interrogation button will appear on the screen.
The lieutenant shows you the place where you must go. It will be marked on your minimap. Again, you must remember to not get to close to the target, otherwise your team can be spotted and the enemies will call for support. This will make the mission much harder. Eliminate your enemies one after another using synchronized shots.
When you kill all enemies, go inside and use action button to open the cell. Order Amaru to follow you and get to the helicopter.
Next, you must transport Amaru to the point on the other side of the mountains. To do that, use a helicopter. Remember that if you want to control it, you must enter from the pilot's side.
First, get some altitude because you have to fly over the mountains. Then, turn the helicopter to face the marked point. The next step is to lean the machine forward with an open throttle. When the helicopter gains a bit of speed, release the lean button. If you are getting closer to the ground, lean the helicopter back to get altitude. Maneuvering like that, get to the marked point.
When you get to the destination point, enter the marked building to complete the first mission.