Arno is thrown into a prison cell. There are a number of things and people you can interact with. If you don't want to do much of this and simply desire to move forward then go to sleep on this mattress to the right of where you start:
For a more detailed experience, you can talk with the man to the left. He is the one sitting against the wall in the room with all the strange symbols:
You can also climb up onto this crate and look out the barred window.
You can also talk with this strange fellow.
You can also head over to the door of the cell. This will have Arno yelling out at the guards.
After that just head for the mattress. There isn't any more to do for now.