At the start Lara don't have a ranged attack till she got the spear. Go to a space and find a spear on the ground so that Lara can pick it up so she can use it later in the game. Tu use this spear, you need to be on the same level as your target and have a clear path. Valid targets will glow white. Although your spear will be gone after thrown, it will result in a one hit kill.