The best choice now is to go back to the room where you started. Open the door ahead and start the action. Bring down all the enemies coming at you in the northern section of the floor.

Find health chalices and plasma gun ammo in this room, and a 1-up behind the secret door. You may want to leave the latter for later.

Find more health in these two rooms.

Find a revolver behind this secret door...

...and more missiles behind this.

If you need more health find some in this room.

Move over here and kill the enemies in this section. Watch the poison in the hallway.

Continue.