moned , you're missing a crucial bit of information; I'll elaborate, since it took me a while to find the pistol too.
When you first start, there's actually a keycard on the table next to you (it's hard to see). That'll open up half the medicine lockers and the door that holds the locker to the pistol. I'll call it Keycard 1.
To get to Keycard 2: Go left and straight down from the starting position, then right and straight through the automated doors and all the way down. Then take a left and straight down, past the enemy and at the dead-end. The keycard is on the floor; it'll open up the other half of the med lockers and the actual locker that holds the pistol.
To get to the pistol itself: With BOTH keycards, go back to the starting area (retracing your steps) and from your starting location, take a right instead, straight through the door, and then take another right all the way down. Open the door on your left with Keycard 1, and the lockers with Keycard 2. The pistol is in one of them, while the other holds a security card that is used to open other doors in the facility.
You can find ammo in random lockers scattered through the area, but I'd suggest getting the lockers right before the automated doors on the path of Keycard 2. For the dev, I'd suggest some sort of color or glowing effect for items that can be picked up, as it's really hard to see the keycards, which are small and nondescript color-wise, especially in the dark areas.