Rosemark on youtube has awesome guides for the game that you can reference to answer questions like this, but to answer your question here -
1. get the screwdriver and unlock the vent in your office
2. Go in the vent and turn left. DO NOT go into the dead end (unless you want a creepy jumpscare and game crash)
3. Take a left just before the dead end and head straight. It should have another locked vent. Use the screwdriver and enter the room.
NOTE: this will unlock a new animatronic for future nights and make them just a little harder. So, I recommend doing this only after you're comfortable with completing the nights (or using Rosemark's guide to get through them).