In order to get extra points, you have to find objects that explode if you shoot them. In the first level, there are a few bonuses. When you get to the tower, shoot it with the grenade launcher. It should fall in a few seconds. When you are facing the stone bridge, shoot at the barrel in the bottom right corner. It should blow up the bridge. When you are in the ship, after the first fight in the fuel room, shoot at the tank you are facing (or the one in the background if you are player one) and it should blow up. Also, any jeeps you shoot will also result in more points.