1 - starting area
2 - ammo stashes
3 - main warehouse
4 - destination (container)
I won't tell you exactly where to go, because besides the main objective you will be allowed to visit three large ammo stashes. It's important to rearm yourself, because the container (main objective) is going to be heavily defended. Start off by using your sniper rifle. You must kill a gunner who is standing on top of the largest warehouse (3 on the map).
I would recommend that you proceed towards the first ammo stash before focusing on the main goal. The stash in the north is the closest one. Use camouflage along the way, however you could also consider surprising enemy soldiers. The supplies are hidden inside the green container (screen). Take whatever you feel is necessary and save your progress.
Go back to the main warehouse (that's where you've killed the gunner). Secure this entire area, activate your cloaking device and find a ladder which leads directly to the roof. Go there now, because you'll want to have a better view of your surroundings.
You shouldn't have any problems while using the ladder, as long as you've secured the area around the main gate. Use camouflage again and proceed to the southern area of the roof. You should be able to spot an enemy tank. Use your rocket launcher to get rid of it (screen).