Welcome to Hell's Maw: the second episode of the game. You'll notice—especially if you're new to Doom and its variants—that when you began the episode, you began the game anew: nothing you accumulated while playing prior episodes has carried over. The game will be over again when you finish this episode. You'll need to find weapons and collect ammo all over again. In the beginning levels of later episodes, "getting a foothold" can be quite difficult until you've gathered weapons and ammo. However, it won't be a problem in this level.
A new enemy
This first level is short and simple. You begin in a medium-sided outdoor area. The Yellow Door is to your left, a closed door to your right, and a doorway to a cave in the center. Clear out the enemies, then go into the cave, get the Ethereal Crossbow, and hit the switch to open the right-hand door outside.
Go to the opened door, and down a passage to a sizeable cave with a lava pool at the far end. You'll encounter a new enemy here: the Weredragon. These guys shoot fireballs that are pretty damaging but not too much trouble to avoid, and for some strange reason drop Ethereal Arrows when killed. Your introduction to them will be in the form of a pretty big fracas. In addition to lots of ammo, you'll find Wings of Wrath in here. Before continuing, go back to the passage and follow the right-hand wall of the cave a few steps until you come to a section of wall that's very slightly lighter than the rest. This is a hidden door to a secret area with a few items.
Activate the Wings and fly over the lava and behind the wall. You'll find a teleporter with the Yellow Key on top. Enter it--you'll pick up the key when you do--and you'll be sent back outside. Unlock the Yellow Door and go down a passage to a second cave mostly filled with lava. To your right, look for a Shadowsphere behind the column. While the Wings are active, use this time to fly around and collect all the ammo and stuff. On one side you'll find a Dragon Claw on a step by the wall. On the other side, on a similar step, is the Green Key. In that pit in the walkway, there's a hidden teleporter in the center that goes back outside. If the Wings have expired, this teleporter will allow you to go for the Quartz Flasks and not become trapped. Look along the wall to the right of the ledge where the Green key was: just above the lava you'll see a section where the texture is different. The wall above is a hidden door to a secret area, with a good item.
Inside the Green Door at the far end of this cave is the level exit. But first, there's a bonus area with some important items. Walk along the walkway across the cave, toward the Green Door. When you pass the narrowest section between the Quartz-Flask pit and the column, it opens two doors. One door is behind the column where the Shadowsphere was, and goes back to the first cave. Go through here and follow the wall to your left. A hidden room along here was also opened. Inside is a new weapon: the Hellstaff. This is a nice weapon: it has a fast firing rate and is highly accurate, although the projectiles travel (like Ethereal Arrows) rather than impacting instantly (like Claw Orbs). It uses Runes, which look like pulsing red symbols. In this room is also a Bag of Holding, which will permanently increase the carrying capacity of ammo for the remainder of the episode. (Subsequent Bags of Holding won't increase capacity further, but will give you a few rounds of most types of ammo.)
Your tasks are now competed. Gather ammo, now that you can carry more you could earlier, then enter the Green Door and exit the level.