Depends on how long they want to keep you there for, I heard some people only stay 3 hours – they like to see you can go to the toilet, eat and drink.
My op was late in the day and I went to a large size so they kept me over night (this was arranged before hand). you literally need yourself, a sports bra and clothing that zips up, nothing that has to go over your head! maybe bring like some sweets/fizzy drink i dno whatever comforts you – I had a fizzy drink waiting for me with my boyfriend haha
It is so exciting, good luck with everything!!!