The first mission works mostly as a tutorial. Whenever there isn’t a score displayed (unless you are on the hardest difficulty) you can do whatever you want, it isn’t going to affect you final score negatively. The only time when the score is activated is in the Mansion Ground Floor. This guide explains how to get through that part on the hardest difficulty, undetected and without killing anyone.
At the end of this guide is a YouTube video with a visual version of this guide.
When you first start walk forward and slightly into the kitchen. This triggers the conversation the cook and the guard have, otherwise you have to wait for it later:
Then keep going down the room you started in. To the left are stairs up. Go to the next floor. Straight across the hallway at the top of the stairs is a bathroom. Enter the bathroom. To the left is a big mirror and there are some sleeping pills standing underneath the mirror. Take the pills:
Head back down to the kitchen. Hide behind the counter until the guard leaves the room. Then go across the kitchen whilst crouching out into the main living area and hide behind the counter there:
There is a coffee cup in front of you which you can put pills into. Do so and then go back around:
Where you exited the kitchen before to the left is a door. Go through that door, but first check if there is still a guard there. If there is wait for the guard to leave. Then follow the room until you can see the main living room again:
Now you have to wait until the head of security goes up to his coffee and drinks it. Then he will come towards you into the room and sit in a chair. Be sure to retreat and hide behind some cover so he can’t see you. Once he is passed out take the key card and hide the body in a closet that is in that same room. Go back up the stairs to the bathroom but instead of going into the bathroom turn right and you will see a walkway and a door with a card swiper next to it:
Go up to the door, swipe the card and then follow that corridor That is the end of the score area, the rest of the mission is basically a cut scene.