Just on the left when you start (linedef #2023), search a berserk.
Just before you grab the shotgun, come in from the east, shoot the small ledge with the chainsaw (linedef #456): a wall will open with the chaingun, boxes of bullets and other goodies.
The pillar near the switch that grants the access to the blue key has a switch (linedef #2013). Teleport then and get a soulsphere.