To get the FIM-92A (Stinger) missile early in the game, in the South American power station, enter the power house and look towards the rear of the building. You will see a window. To the left of that is a metal wall computer with two blinking lights. Destroy it (try using C4). You will see a cinematic of the power ending and all is quiet. Run out the window and climb the ladder. Once at the top, shimmy to your left across the only wire there, then drop down and pick up the missile, along with your rations and noodles. It will be useful for the air support you will encounter.