After the first teleporter, you'll find yourself in a room made of blood and guts. Go to the righthand pillar that's closest to the building. Push the button on the back of the pillar, then turn around and enter the cavern.
After you get through the bridgework over the lava, you'll be in a place with a blood river and a baron of hell. Jump off the left side of the path and hurry into either one of the openings you see to find some rockets and a switch.
When you stand next to the exit portal, there's a waterway in front of you. Jump into this waterway's lefthand side and go straight to telefrag a chaingunner and get some blue armour.
After you push the button to lower the red key, go back to the lift in the same building. Look to the left of the lift to find a plasma gun and some cell packs.