As I have already mentioned, the first floor is inaccessible to the guests. It is going to be much easier to overcome the obstacle if you have taken the door on the patio outside - the ones that were guarded by a single guard- because this will get you in one of the three bedrooms on the first floor. Otherwise, you will have to go creative. The easiest method is to choose the staircase for the servants that's adjacent to the ballroom (i.e. the one that you might have already used to get down to the basement). The staircase is guarded by a single guard, but if you wait, he will walk away, and you can go upstairs. To reach one of the bedrooms.
Another option is possessing a rat or a human. In the case of a rat, you would have to find a ventilation shaft that leads up to the floor (the above screenshot). As far as a human is concerned, you would have to cross the wall of light in the foyer, after possessing them, and take the stairs.
The last method consists in disabling the wall of light, which is, unfortunately, quite complicated. The power supply for the wall of light is located to the right of it, and the door to it is locked by default. As a result, you would have to steal the whale oil closet key from one of the guards in the foyer. After you obtain the key, unlock the access to the side room and rewire the wall of light, or disable it by removing the whale oil tank.
The Estate's first floor is patrolled by several guards, so you need to be extremely careful. If you plan on eliminating the guards, make sure that you carry away the bodies into secluded areas out of anybody's sight. I do not recommend stashing them in the toilet on the floor, because the guards like to visit it frequently. To stash the bodies, it is better to use bedrooms. I recommend that you seize the opportunity to loot the bodies, because one of them has the Waverly's bedroom key on him.
Regardless of whether you eliminated the guards or not, start exploring the rooms on the floor. In the abovementioned toilet there is gold. In the central room with the cabinets apart from valuables, you can find Sokolov's painting (the above screenshot). The last one of the less important rooms is the library. In here, you can find two books.
Lydia Boyle's bedroom (the above screenshot) is located above the foyer. In here you can find gold (you definitely need to collect pearls from both chests!), basement vault key and Lydia Boyle's Diary.
Waverly Boyle's bedroom is locked by default and you can access it through the attic (this was described further in the text) or by acquiring Waverly's bedroom key from one of the nearby guards. Inside the bedroom, there are gold, food, a copy of Waverly's bedroom key and Waverly Boyle's Diary.
Esma Boyle's bedroom is the one that you can access from the balcony outside and from the service staircase. You can find here gold, food, Sokolov's painting, Dunwall Tower skeleton key, Piero's spiritual remedy, Esma Boyle's Diary and Note to Esma Boyle, whose reading is a way of identifying your target (more about it in the subsection entitled Identify your target).
To wind up, it is worth mentioning the attic between Waverly's and Esma Boyle's bedrooms. In both of them there is a red rope (the above screenshot), whose pulling unlocks the access to the attic. It's worth going there, because you will find a lot of valuables in there.