If you read that in the voice of Jim Carrey as "The Grinch", you and I are going to get along just fine.
But in all seriousness, dress COMFY. I'm talking slippers, oversized sweatpants & sweatshirt, etc. You don't even have to wear a bra! In fact, I encourage you not to in order to avoid any harsh lines or indents on your skin.
You bring yourself! A water or something to drink, maybe a snack or two, and if it makes you feel better you can even bring a friend! As far as packing goes, if you have a couple of outfits you like, throw them in the bag too! I always tell everyone, I'd rather you bring it and not need it than not bring it and wish you had it. That goes for anything....heels, accessories, personal items, a veil if you're getting married soon, etc. Bring it all but the kitchen sink or don't - your choice!
Nope! Unless you have a remaining balance. In that case, the remaining balance of your session will be due when you arrive (or before you leave). I would prefer the balance be paid in cash but if you can't make it to the bank and you don't have cash on you - Venmo is just fine.