@JulieSav You've had a lot of good advice here. I would add my voice to having a photo taken beforehand. I didn't before I went in for the wide excision and wished I had, but did before I had the mastectomy. You can always delete it later if you decide that it makes you feel uncomfortable.
Going in to hospital - take some face wipes because it's easier than washing your face every 20 minutes (always feel so greasy in hospital). If your hospital has a garden, take some footwear that you can slip on over TEDs and a reasonable robe - my surgeon was very happy for me to take the lift down and wander through the hospital garden rather than do laps of the corridors and it's so much better for your state of mind to get some fresh air. I also took some drinks that I prefer rather than drink hospital tea/coffee/cordial all of the time. Another thing that can be useful is a shawl or pashmina as the airconditioning can get quite chilly in hospital.
I was given one of the Berlei Brunnhilde bras which I have to say that I found dreadfully uncomfortable and a bit upsetting as I'd never worn anything that looked so much like a "foundation garment". You don't say where you are but I also had someone come in to the hospital who fitted and sold bras (I still get bras from her as it is such a great service). You may find that for the early days, a pull-on crop bra with a soft-form is all you want - about $15 for two from Kmart of Best and Less. I actually went braless as much as possible in those first weeks. And try to have a couple of comfortable front-opening tops for when you get home. It's quite possible to get stuck trying to get clothing over your head as your movement can be limited (I speak from experience as my poor teenage son had to come to my rescue when I couldn't get a t-shirt off).
Believe it or not, you will get through the surgery and out the other side. Bits of it can be challenging but for most of us it has been a relief to get the cancer out. All the best for Wednesday.