most useful thing in my experience would have been the tiny details of how to contact people etc - not just 'see the office' or 'see Mrs Jones' or whatever, but how. Main reception was separaate and then there was this open-plan office nearby with various people who dealt with different things, and it terrified me, not knowing whether to knock and wait, just to go in, how to get the attention of the right person, etc. Same with things like the music room - you might want to see the music teacher, but you had to go into a room where older kids were playing instruments or hanging around or looking like they belonged and were doing stuff, to see if the music director was in an office or something. Were you supposed to just go in and interrupt all of that, or wait, or what.
Details of where you'd go and how to sign up for clubs, where the lost and found was, what the system for choosing/paying for food was, who was allowed to go where at what times - canteen, field, playground, benches etc, sometimes seemed reserved for certain year groups.
Something from older students with a reassuring message, maybe telling them the 'unofficial' way things might work
I was so nervous about making a fool of myself, being in the wrong place, not knowing how to 'go in' to rooms and offices, where I was allowed to be, etc.