From the starting area, after getting briefed and the gate opens after the buzzer, crawl through the only vent leading to the next hallway.
Achievement Unlocked: Off to a Good Start
You'll unlock this achievement automatically as soon as you crawl out of the vent
After climbing out the vent, look for the two wires connected to the breakers. They should take you to the elevator that will lead to the lower floor. You need to find and solve the following puzzles (click on their respective pages for the detailed explanation on how to clear them):
After solving them, you can also explore the area more to find the Gymnasium, a special room which can unlock an achievement for you. In subsequent playthroughs, you don’t need to bother with that room anymore.
Once you have activated the two breakers in the Library and Looping Hallway areas, follow the red wire to the exit gate (which is in the starting area, in the room front of the vent where you crawled earlier) and follow it to the elevator.